Item that's spare? Good! (5)
I believe the answer is:
thing
'item' is the definition.
(both can mean an object)
'spare? good' is the wordplay.
'spare?' becomes 'thin' (similar in meaning).
'good' becomes 'g' (abbreviation).
'thin'+'g'='THING'
'that's' acts as a link.
(Other definitions for thing that I've seen before include "Object, artefact" , "Object; gadget" , "cup of tea" , "Affair - matter" , "Individual unit" .)