Popular act without doubt (6)
I believe the answer is:
indeed
'without doubt' is the definition.
(I've seen this before)
'popular act' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'act' becomes 'deed' (deed is a kind of act).
'in'+'deed'='INDEED'
(Other definitions for indeed that I've seen before include "Unquestionably" , "Really - what a surprise!" , "Really - truly" , "In reality" , "Truly or admittedly" .)