Best priest drops down for mass (4)
I believe the answer is:
lump
'for mass' is the definition.
(I have seen 'Shapeless mass' mean 'lump' so perhaps 'mass' could also mean 'lump')
'best priest drops down' is the wordplay.
'best' becomes 'plum' (informal term for best or top-class).
'priest' becomes 'p' (ecclesiastical abbreviation).
'drops' indicates named letters should be taken away.
'down' becomes 'p' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'plum' with 'p' removed is 'lum'.
'lum'+'p'='LUMP'
Can you help me to learn more?
(Other definitions for lump that I've seen before include "Hunk - chunk" , "growth" , "Gross" , "Node" , "A dollop" .)