Mark working exactly right (4,2)
I believe the answer is:
spot on
'exactly right' is the definition.
(I've seen this before)
'mark working' is the wordplay.
'mark' becomes 'spot' (spotting is a kind of marking).
'working' becomes 'on' (e.g. a machine is working or on).
'spot'+'on'='SPOT-ON'
(Other definitions for spot on that I've seen before include "Completely accurate" , "perfect" , "Precisely what's required" , "Absolutely right" , "What the leading actor may have" .)