Old priest involved in religious controversy (3)
I believe the answer is:
eli
'old priest' is the definition.
(Biblical character)
'involved in religious controversy' is the wordplay.
'involved in' indicates the answer is hidden within the clue.
'ELI' is hidden within 'religious controversy'.
(Other definitions for eli that I've seen before include "Man's name" , "OT high priest" , "Distort then untruth to get a Biblical character" , "Samuel's teacher (OT)" , "Old Testament priest, the prophet Samuel's teacher" .)