Top off, cook (5)
I believe the answer is:
outdo
'top' is the definition.
(both can mean to surpass or beat)
'off cook' is the wordplay.
'off' becomes 'out'.
'cook' becomes 'do' (do can mean to cook).
'out'+'do'='OUTDO'
(Other definitions for outdo that I've seen before include "Surpass, go one better than" , "going to top?" , "Surpass someone at something" , "Br superior to (someone)" , "Be better in performance (than another person)" .)