Vote in case of reasonable performance (6)
I believe the answer is:
return
'vote in' is the definition.
'case of reasonable performance' is the wordplay.
'case of' means to remove the middle letters (outsides of).
'performance' becomes 'turn' (turn is a kind of performance**).
'reasonable' with its centre taken out is 're'.
're'+'turn'='RETURN'
(Other definitions for return that I've seen before include "Come back with a formal report on transactions" , "Proceeds" , "reply" , "Send, bring or come back" , "get back to original state" .)