Former Derry City man Killian Brennan came back to haunt his old club and fire St Patrick's Athletic back to the top of the league.
The talented midfielder's early free-kick was enough for the Inchicore men to leap-frog Dundalk and move into pole position with seven games remaining.
The title chasers took the lead on three minutes thanks to a wonder strike from Brennan.
The ex-Bohemians man curled a stunning 25-yard free-kick high into Gerard Doherty's top right-hand corner, after Shane McFaul was brought down by Shane McEleney on the edge of the box.
The Dubliners were full of confidence and their neat passing had the home side chasing shadows. Just after the hour mark the hosts sparked into life with Pat Kavanagh going close but his left-footed strike from 25 yards was saved by Clarke.
Unfortunately for Liam Buckley's men Stephen Maher was stretchered off following a long stoppage and was replaced by Sean Gannon.
Derry City – Doherty, Madden, S McEleney, McBride, McNamee; Higgins, Molloy; P McEleney (Kavanagh 42), McDaid, Griffin (Curran h-t); Patterson.
St Patrick's Athletic – Clarke, O'Brien, Kenna, Browne, Bermingham; Byrne, McFaul (Maher 76, Gannon 89), Bogler, Brennan; Fagan (Flood 68), Kavanagh.
Ref – T Connolly (Dublin).