A putty one isn't exactly metal! (5)
I believe the answer is:
medal
'a putty one' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'isn't exactly metal' is the wordplay.
'isn't exactly' becomes 'med' (I am not sure about this - if you are sure you should believe this answer much more).
'metal' becomes 'Al' (chemical symbol for aluminium).
'med'+'al'='MEDAL'
Can you help me to learn more?
(Other definitions for medal that I've seen before include "Decoration, award" , "Lamed (anag)" , "Award for achievement" , "Award for distinction" , "Prize disc for as for Olympic Games win" .)