Old theologian, peculiar (3)
I believe the answer is:
odd
'peculiar' is the definition.
(thesaurus)
'old theologian' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'theologian' becomes 'dd' (Doctor of Divinity degree).
'o'+'dd'='ODD'
(Other definitions for odd that I've seen before include "Number not divisible by two" , "unlike 1984" , "Rum" , "Peculiar or unpaired" , "Extraordinary" .)