Popular part of song? The opposite (7)
I believe the answer is:
inverse
'the opposite' is the definition.
(synonyms)
'popular part of song?' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'part of song?' becomes 'verse' (I've seen this before).
'in'+'verse'='INVERSE'
(Other definitions for inverse that I've seen before include "Reversed in position like enviers" , "Upside down - opposite in effect" , "Antonym" , "The exact opposite of enviers" , "Something the exact opposite of another" .)