Priests holding mass for religious feast (6)
I believe the answer is:
lammas
'religious feast' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'priests holding mass' is the wordplay.
'priests' becomes 'lamas' (lama is a kind of priest).
'holding' means one lot of letters goes inside another.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'lamas' placed around 'm' is 'LAMMAS'.
'for' is the link.
Can you help me to learn more?
(Other definitions for lammas that I've seen before include "harvest celebration of yore" , "summer feast" , "Ould ... Fair takes place in August in Co Antrim" , "old festival" , "harvest festival" .)