Conservative yarn, not the first (4)
I believe the answer is:
tory
'conservative' is the definition.
(informal term for a Conservative Party member)
'yarn not the first' is the wordplay.
'yarn' becomes 'story' (synonyms).
'not the first' means to remove the first letter.
'story' with its initial letter taken away is 'TORY'.
(Other definitions for tory that I've seen before include "type on right? Yes" , "Traditionalist" , "blue" , "A British Conservative" , "Politician; tyro (anag.)" .)