Engaged by popular application (2-3)
I believe the answer is:
in-use
'engaged' is the definition.
(synonyms)
'by popular application' is the wordplay.
'by' means one lot of letters go next to another.
'popular' becomes 'in' ('in' can mean trendy).
'application' becomes 'use' (I've seen this in another clue).
'in'+'use' is 'IN-USE'.
(Other definitions for in-use that I've seen before include "Unavailable; busy" , "Being employed -- unavailable" , "Engaged" , "In employment or practice" , "Occupied" .)