The visitors looked set to snatch the win despite being second best throughout before succumbing to the home side's pressure with a goal through Clarke. It looked as if it wasn't going to come, but the on-loan Town striker popped up with seconds remaining.
Gary Smith, Karleigh Osborne and Sam Williams combined to set up David Hunt and his low cross was turned home at the far post by Clarke for the last-gasp leveller.
The young forward was villain-turned-hero as he had earlier seen a penalty, awarded for hand-ball by Matt Gill, saved low to his left by Exeter goalkeeper Paul Jones.
Click here for a Player Profile on Billy Clarke or Click here to see a game by game history of his season to date.