This is a reversal clue - look for indicators like 'back', 'returned', 'up' (in down clues), or 'west' (in across clues)
Break down the clue into definition and wordplay parts
Answer: FADDIST | Wordplay: Duke defending a theologian, one given to temporary obsession? (7)A DD (Doctor of Divinity, theologian) in FIST (duke, as in “put your dukes up”)