Idiot in religious service losing head (3)
I believe the answer is:
ass
'idiot' is the definition.
(thesaurus)
'religious service losing head' is the wordplay.
'religious service' becomes 'mass' (I've seen this before).
'losing head' means to remove the first letter.
'mass' with its initial letter taken off is 'ASS'.
'in' acts as a link.
(Other definitions for ass that I've seen before include "Nincompoop" , "Hardy and sure-footed animal" , "Bottom, temporarily" , "Horse-type animal" , "'The law is a - - - a idiot' (Mr Bumble)" .)