Key to winning a trophy (3)
I believe the answer is:
cup
'a trophy' is the definition.
(cup is a kind of trophy)
'key to winning' is the wordplay.
'key' becomes 'c' (I've seen this in other clues**).
'to winning' becomes 'up' (I've seen this in other clues).
'c'+'up'='CUP'
(Other definitions for cup that I've seen before include "Beaker" , "Container to drink from - prize" , "Trophy; drinking vessel" , "Award; mug" , "McCarthy, Walker or Maguire" .)