The object is to get a spare key (5)
I believe the answer is:
thing
'the object is to' is the definition.
I can't tell whether this defines the answer.
'a spare key' is the wordplay.
'a spare' becomes 'thin' (I've seen this in another clue).
'key' becomes 'g' (musical key).
'thin'+'g'='THING'
'get' acts as a link.
Can you help me to learn more?
(Other definitions for thing that I've seen before include "cup of tea" , "An unnamed object or item" , "Inanimate object" , "Body" , "Night (anag)" .)