Swell theologian holds an enemy back (5)
I believe the answer is:
dandy
'swell' is the definition.
(I know that swell can be written as dandy)
'theologian holds an enemy back' is the wordplay.
'theologian' becomes 'DD' (Doctor of Divinity degree).
'holds' means one lot of letters goes inside another.
'back' says to take the final letters (the letter at the back).
The final letter of 'enemy' is 'y'.
'dd' going around 'an' is 'dand'.
'dand'+'y'='DANDY'
(Other definitions for dandy that I've seen before include "Fine for American" , "Foppish fellow" , "Man about town" , "Playboy" , "Showy dresser" .)