Gathered to return in song (5)
I believe the answer is:
motet
'song' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see a link between them that I can't see?
'gathered to return in' is the wordplay.
'gathered' becomes 'met' ('meet' can be a synonym of 'gather').
'return' shows that the letters should be reversed in order.
'in' is an insertion indicator.
'to' reversed gives 'ot'.
'met' placed around 'ot' is 'MOTET'.
Can you help me to learn more?
(Other definitions for motet that I've seen before include "Short piece of church choral music" , "Religious choral composition" , "Unaccompanied choral composition with sacred lyrics" , "10 [COMPOSITION]" , "Musical composition" .)