Place occupied by duke or knight's spouse (4)
I believe the answer is:
lady
'knight's spouse' is the definition.
(I've seen this in another clue)
'place occupied by duke' is the wordplay.
'place' becomes 'lay' (synonyms).
'occupied by' means one lot of letters goes inside another.
'duke' becomes 'd' (abbreviation).
'lay' enclosing 'd' is 'LADY'.
'or' is the link.
(Other definitions for lady that I've seen before include "Luck, for example" , "Lord's wife - peeress's title" , "High-class woman" , "See 19" , "Peeress" .)