Collaborator with police popular earlier (8)
I believe the answer is:
informer
'collaborator with police' is the definition.
(as in a police informant)
'popular earlier' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'earlier' becomes 'former'.
'in'+'former'='INFORMER'
(Other definitions for informer that I've seen before include "Criminal helping police" , "One snitching to police" , "Whistle-blower" , "sneak" , "(Maybe super)grass" .)