Song in church, a new song endlessly going on (7)
I believe the answer is:
chanson
'song' is the definition.
(I have seen 'French song' mean 'chanson' so perhaps 'song' could also mean 'chanson')
'church a new song endlessly going on' is the wordplay.
'church' becomes 'ch' (abbreviation on maps etc).
'new' becomes 'n' (abbreviation as in New Testament NT).
'endlessly' means to remove the last letter.
'going on' means one lot of letters go next to another.
'song' with its last letter taken off is 'son'.
'ch'+'a'+'n'+'son'='CHANSON'
'in' acts as a link.
(Other definitions for chanson that I've seen before include "song for Piaf?" , "(French) song" , "A number" .)