Idiot from Trinity (3)
I believe the answer is:
nit
'idiot' is the definition.
(nit can mean a stupid person)
'from trinity' is the wordplay.
'from' indicates the answer is hidden within the clue.
'NIT' is hidden within 'trinity'.
(Other definitions for nit that I've seen before include "Foolish person" , "Parasite's egg made of tin" , "no egghead?" , "Egg of a louse (3)" , "Egg; fool" .)