Song encountered, to slot back in (5)
I believe the answer is:
motet
'song' is the definition.
Both the definition and answer are singular nouns.
Perhaps they are linked in a way I don't understand?
'encountered to slot back in' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'back' suggests the final letters.
'in' means one lot of letters goes inside another.
The last letters of 'to slot' are 'ot'.
'met' going around 'ot' is 'MOTET'.
Can you help me to learn more?
(Other definitions for motet that I've seen before include "Short sacred choral work" , "Piece of sacred music" , "Short sacred choral piece" , "Polyphonic choral liturgical piece" , "Church composition - totem (anag)" .)