Old-fashioned theologian dined within (5)
I believe the answer is:
dated
'old-fashioned' is the definition.
(dated can mean old-fashioned)
'theologian dined within' is the wordplay.
'theologian' becomes 'DD' (Doctor of Divinity degree).
'dined' becomes 'ate' (I've seen this in another clue).
'within' indicates putting letters inside.
'dd' placed around 'ate' is 'DATED'.
(Other definitions for dated that I've seen before include "Walked out with" , "old > saw" , "not without romantic engagements" , "Met socially by arrangement" , "Out of fashion" .)