Decline a prize (7)
I believe the answer is:
atrophy
'decline' is the definition.
(I've seen this before)
'a prize' is the wordplay.
'prize' becomes 'trophy' ('trophy' can be a synonym of 'prize').
'a'+'trophy'='ATROPHY'
(Other definitions for atrophy that I've seen before include "Waste away - sounds like a sporting award" , "shrivel" , "Decay" , "Degeneration" , "Degenerate" .)