Priest taking part in ritual, a mass (4)
I believe the answer is:
lama
'priest' is the definition.
(lama is a kind of priest)
'part in ritual a mass' is the wordplay.
'part in' says the answer is hidden in the clue.
'LAMA' is hidden in the letters of 'ritual a mass'.
'taking' is the link.
(Other definitions for lama that I've seen before include "Religious leader" , "A Tibetan Buddhist priest or monk" , "Holy man in Tibet" , "A Buddhist priest in Tibet (4)" , "A Buddist monk in Tibet" .)