Queen sees king hiding in tree (4)
I believe the answer is:
mary
'queen' is the definition.
(Mary I or Mary II)
'king hiding in tree' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'hiding in' is an insertion indicator.
'tree' becomes 'may' (I've seen this in other clues).
'r' placed within 'may' is 'MARY'.
'sees' acts as a link.
(Other definitions for mary that I've seen before include "Mother of Jesus" , "a religious icon" , "See 2" , "Successor to Edward VI" , "Person venerated by Christians" .)