Slip, perhaps, if climbing tree (7)
I believe the answer is:
fielder
'slip' is the definition.
(cricket term)
'if climbing tree' is the wordplay.
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'tree' becomes 'elder' (elder is a kind of tree).
'if' in reverse letter order is 'fi'.
'fi'+'elder'='FIELDER'
'perhaps' acts as a link.
(Other definitions for fielder that I've seen before include "Slip or cover, maybe" , "One as cover, slip" , "one in gully, perhaps" , "Cricketing slip, say?" , "cover, perhaps" .)