Object made of spare string (5)
I believe the answer is:
thing
'object' is the definition.
(thing is a kind of object)
'spare string' is the wordplay.
'spare' becomes 'thin' ('thin' can be similar in meaning to 'spare').
'string' becomes 'g' (**).
'thin'+'g'='THING'
'made of' is the link.
(Other definitions for thing that I've seen before include "Affair - matter" , "Obsession" , "A creature" , "Individual unit" , "Unspecified object" .)