Popular old rat (8)
I believe the answer is:
informer
'rat' is the definition.
(synonyms)
'popular old' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'old' becomes 'former' (I've seen this before).
'in'+'former'='INFORMER'
(Other definitions for informer that I've seen before include "stoolie" , "Criminal helping police" , "Grass, nark, canary" , "who may betray someone?" , "spy" .)