Unfashionable theologian dined inside (5)
I believe the answer is:
dated
'unfashionable' is the definition.
(similar in meaning)
'theologian dined inside' is the wordplay.
'theologian' becomes 'DD' (Doctor of Divinity degree).
'dined' becomes 'ate' (I've seen this in another clue).
'inside' indicates putting letters inside.
'dd' placed around 'ate' is 'DATED'.
(Other definitions for dated that I've seen before include "Met socially by arrangement" , "Passe" , "Out of fashion" , "Old fashioned" , "Went out with - old hat" .)