Feeble golf is an obsession (5)
I believe the answer is:
thing
'an obsession' is the definition.
(as in 'having a thing' for someone or something)
'feeble golf' is the wordplay.
'feeble' becomes 'thin'.
'golf' becomes 'g' (phonetic alphabet: alpha, bravo, charlie etc.).
'thin'+'g'='THING'
'is' is the link.
(Other definitions for thing that I've seen before include "Inanimate object" , "Night (anag)" , "Affair - matter" , "Unspecified object" , "Body" .)