Graduate hiding in tree, beneath king’s citadel (6)
I believe the answer is:
kasbah
'citadel' is the definition.
(term for a North African citadel)
'graduate hiding in tree beneath king's' is the wordplay.
'graduate' becomes 'BA' (someone with a Bachelor of Arts degree).
'hiding in' indicates putting letters inside.
'tree' becomes 'ash tree' (ash is a kind of tree).
'beneath' says to put letters next to each other (in a down clue).
'king' becomes 'K' (abbreviation for king in chess).
'ba' put into 'ash' is 'asbah'.
'asbah' after 'k' is 'KASBAH'.
(Other definitions for kasbah that I've seen before include "Citadel in, for example, Algiers" , "Citadel or quarter of North African cities" , "Citadel of a North African city" , "Algerian fortress" .)