Really popular female performance (2,4)
I believe the answer is:
in fact
'really' is the definition.
(synonyms)
'popular female performance' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'female' becomes 'f' (abbreviation).
'performance' becomes 'act' (act is a kind of performance).
'in'+'f'+'act'='IN FACT'
(Other definitions for in fact that I've seen before include "really!" , "Actually" , "'In reality (2,4)'" .)