Really popular act (6)
I believe the answer is:
indeed
'really' is the definition.
(synonyms)
'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 "Very much so" , "Absolutely" , "still" , "Word expressing surprise, disbelief, etc" , "In truth, in fact" .)