Feeble golf becomes an obsession (5)
I believe the answer is:
thing
'an obsession' is the definition.
(I've seen this in other clues)
'feeble golf' is the wordplay.
'feeble' becomes 'thin'.
'golf' becomes 'g' (phonetic alphabet: alpha, bravo, charlie etc.).
'thin'+'g'='THING'
'becomes' acts as a link.
(Other definitions for thing that I've seen before include "An unnamed object or item" , "Night (anag)" , "Individual object" , "Inanimate object" , "Obsession" .)