Outperform, say, forty (5)
I believe the answer is:
excel
'outperform' is the definition.
'say forty' is the wordplay.
'say' indicates a 'sounds like' (homophone) clue.
'forty' becomes 'xl' (Roman numerals).
'xl' sounds like 'EXCEL'.
(Other definitions for excel that I've seen before include "Show brilliance" , "Be pre-eminent" , "Surpass; shine" , "Be very good at, better than others" , "Succeed, stand out" .)