Popular application's not free? (2,3)
I believe the answer is:
in-use
'not free?' is the definition.
(I've seen this in another clue)
'popular application's' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'application' becomes 'use' (I've seen this in another clue).
'in'+'use'='IN-USE'
(Other definitions for in-use that I've seen before include "Currently employed" , "Engaged" , "Being employed -- unavailable" , "In employment or practice" , "Occupied" .)