A 31st minute strike from Nacewa meant that the sides were on level terms (6-6) at half-time, and as a result, the province were still on course to leapfrog table toppers Connacht.
Yet, a strong start to the second period from the Warriors yielded another six-point salvo from influential out-half Clegg, and this was enough to give the defending league champions a much-needed triumph.
Following consecutive victories over Munster, the Newport Gwent Dragons and Cardiff Blues, Glasgow were certainly a dangerous proposition for tonight’s visitors. However, it was Leinster who made the brighter start to the proceedings, as Nacewa successfully converted a 35-metre penalty to edge his side into a slender lead.
Excellent breaks by James Tracy and Cathal Marsh initially kept Glasgow on the back foot, but Gregor Townsend’s charges finally settled into their stride. Clegg, who had rejoined the Scottish outfit in advance of this rearranged game, split the posts from a 12th minute penalty, and he doubled his tally with another three-pointer just shy of the 20 minute mark.
Pressure from Glasgow did force some errors from Leinster, but with 27 minutes on the clock, lively centre Garry Ringrose kickstarted a promising attack with an eye-catching chip on the left wing.
A wayward Cathal Marsh pass denied Nacewa the opportunity to break through on the far flank, but he eventually restored parity courtesy of his second effort on goal. The skipper was presented with an ideal chance to move Leinster ahead again at the end of the first half, but he was uncharacteristically off-target from the tee.
Tellingly, for much of the third quarter, it was the Warriors who provided the greater attacking edge. The Leinster defence came under increasing pressure and they leaked a close range penalty, ten minutes in. Clegg comfortably sent the ball through the uprights once more, and the English-born kicker soon widened the margin to six points from a left hand angle.
Winger Lee Jones had earlier threatened to break over at the end of a lung-bursting run from deep. Heading into the final quarter, Ross Molony and Leinster ‘A’ captain Peadar Timmins were introduced up front by Cullen as the province attempted to address their deficit.
They were joined from the bench by debutant out-half Joey Carbery in the latter stages, but despite applying some pressure close to the Glasgow whitewash, Leinster had to settle for a losing bonus point which moves them just one point behind Connacht ahead of the sides’ derby clash in Galway on Saturday week.
GLASGOW WARRIORS SCORERS: R. Clegg (4 penalties)
LEINSTER SCORERS: I. Nacewa (2 penalties)
GLASGOW WARRIORS: Peter Murchie CAPTAIN; Lee Jones, Mark Bennett, Sam Johnson, Rory Hughes; Rory Clegg, Grayson Hart; Gordon Reid, Fraser Brown, Sila Puafisi, Greg Peterson, Leone Nakarawa, James Eddie, Simone Favaro, Adam Ashe.
REPLACEMENTS: James Malcolm, Jerry Yanuyanutawa, Zander Fagerson, Scott Cummings, Tyrone Holmes, Ali Price, Nick Grigg, Glenn Bryce.
LEINSTER:
15. Zane Kirchner
14. Isa Nacewa CAPTAIN
13. Garry Ringrose (Noel Reid, 76)
12. Ben Te’o
11. Dave Kearney
10. Cathal Marsh (Joey Carbery, 76)
9. Luke McGrath (Isaac Boss, 73)
1. Peter Dooley (Jeremy Loughman, 73)
2. James Tracy
3. Tadhg Furlong (Michael Bent, 73)
4. Mick Kearney (Peadar Timmins, 64)
5. Hayden Triggs (Ross Molony, 54)
6. Dominic Ryan
7. Dan Leavy
8. Jordi Murphy
REPLACEMENT NOT USED: Bryan Byrne.
REFEREE: George Clancy (IRFU); ASSISTANT REFEREES: Lloyd Linton, Mike Adamson (both SRU); CITING COMMISSIONER: Douglas Hunter (SRU); TMO: Jim Yuille (SRU).