Champion leading in golf, say (7)
I believe the answer is:
support
'champion' is the definition.
(I've seen this before)
'leading in golf say' is the wordplay.
'leading' becomes 'up' ('up' can be similar in meaning to 'leading').
'in' means one lot of letters goes inside another.
'golf say' becomes 'sport' (golf is a kind of sport).
'up' going within 'sport' is 'SUPPORT'.
(Other definitions for support that I've seen before include "Provide with food, clothing and shelter, or encouragement" , "Uphold" , "used to give strength?" , "Bolster or back up" , "Hold up" .)