Do the job OK but leave little margin for error (3,2,4)
I believe the answer is:
cut it fine
'leave little margin for error' is the definition.
'do the job ok' is the wordplay.
'do the job' becomes 'cut it' (I've seen this in another clue).
'ok' becomes 'fine' (synonyms).
'cutit'+'fine'='CUT IT FINE'
'but' acts as a link.
(Other definitions for cut it fine that I've seen before include "Achieve something at the last possible moment" , "Allow little margin of time, space etc" , "Take risks" , "only just make it" , "Almost be too late" .)