Indulge in golf with light keep fit, climbing (7)
I believe the answer is:
gratify
'indulge' is the definition.
(I know that indulge can be written as gratify)
'golf with light keep fit climbing' is the wordplay.
'golf' becomes 'g' (phonetic alphabet: alpha, bravo, charlie etc.).
'with' is a charade indicator (letters next to each other).
'light' becomes 'ray' (ray is a kind of light).
'keep' indicates putting letters inside.
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'fit' backwards is 'tif'.
'ray' enclosing 'tif' is 'ratify'.
'g'+'ratify'='GRATIFY'
'in' is the link.
(Other definitions for gratify that I've seen before include "Indulge (a desire)" , "Please - indulge" , "Give pleasure to" .)