Support for young boxer? (4,2)
I believe the answer is:
prop up
'support' is the definition.
(propping up is a kind of supporting)
'for young boxer?' is the wordplay.
'for' becomes 'pro' (as in being 'pro' or in favour of something).
'young boxer?' becomes 'pup' (boxer is a type of dog).
'pro'+'pup'='PROP UP'
(Other definitions for prop up that I've seen before include "Support" , "Underpin" , "Give financial backing" , "Keep going" .)