Cap off cook (5)
I believe the answer is:
outdo
'cap' is the definition.
(I've seen this before)
'off cook' is the wordplay.
'off' becomes 'out' (both can mean wrong or inaccurate).
'cook' becomes 'do' (do can mean to cook).
'out'+'do'='OUTDO'
(Other definitions for outdo that I've seen before include "Perform better than surpass" , "Defeat" , "Perform better than, be superior to" , "Overcome" , "Cap" .)