Exasperated compiler's utterance: 'Start again!' (4,2,6,3)
I believe the answer is:
back to square one
'start again' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
(Other definitions for back to square one that I've seen before include "Ready to start all over again" , "no further forward?" , "Illegal pawn move made" , "Start from scratch" , "having another go at solving 1 ac?" .)