Join a religious man (3)
I believe the answer is:
add
'join' is the definition.
(thesaurus)
'a religious man' is the wordplay.
'religious man' becomes 'DD' (abbreviation for Doctor of Divinity degree).
'a'+'dd'='ADD'
(Other definitions for add that I've seen before include "Combine (numbers)" , "Sum, put together" , "Tot; append" , "Aggregate or supplement" , "Reckon" .)