Popular act? Absolutely (6)
I believe the answer is:
indeed
'absolutely' is the definition.
(both can be used to agree with something)
'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 "Really - truly" , "Truthfully, really" , "Very much so" , "In reality" , "Absolutely" .)