Supporter in a cap (7)
I believe the answer is:
abetter
'supporter' is the definition.
(I've seen this before)
'a cap' is the wordplay.
'cap' becomes 'better' (I've seen this before).
'a'+'better'='ABETTER'
'in' acts as a link.
(Other definitions for abetter that I've seen before include "person giving encouragement" , "person to give encouragement?" , "One who encourages" , "Supporter" , "he helps you in trouble" .)