Popular, one-time whistle-blower (8)
I believe the answer is:
informer
'whistle-blower' is the definition.
(I've seen this before)
'popular one-time' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'one-time' becomes 'former' (I've seen this in another clue).
'in'+'former'='INFORMER'
(Other definitions for informer that I've seen before include "(Maybe super)grass" , "spy" , "One telling" , "Grass, nark, canary" , "stoolie" .)