Champion winning in tennis, perhaps (7)
I believe the answer is:
support
'champion' is the definition.
(I've seen this before)
'winning in tennis perhaps' is the wordplay.
'winning' becomes 'up' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'tennis perhaps' becomes 'sport' (tennis is a kind of sport).
'up' placed into 'sport' is 'SUPPORT'.
(Other definitions for support that I've seen before include "used to give strength?" , "Give assistance to" , "Bear the weight of (something)" , "Hold up" , "Bear; corroborate" .)