Delightful singing by chaps leaving Mass (10)
I believe the answer is:
enchanting
'delightful' is the definition.
(I've seen this before)
'singing by chaps leaving mass' is the wordplay.
'singing' becomes 'chanting' (chanting is a kind of singing).
'by' is a charade indicator (letters next to each other).
'chaps' becomes 'men' (chap can mean a man).
'leaving' suggests deleting specific letters.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'men' with 'm' taken out is 'en'.
'chanting' put after 'en' is 'ENCHANTING'.
(Other definitions for enchanting that I've seen before include "Beguiling" , "occupation Circe undertook" , "Bewitching" , "Charming" , "Delightful" .)