Speculate about the Queen's headgear (5)
I believe the answer is:
beret
'headgear' is the definition.
(beret is a kind of headgear)
'speculate about the queen's' is the wordplay.
'speculate' becomes 'bet' (I've seen this in other clues).
'about' means one lot of letters goes inside another.
'the queen' becomes 'er' (Elizabeth Regina).
'bet' enclosing 'er' is 'BERET'.
(Other definitions for beret that I've seen before include "Flattish round hat" , "Flattish felt cap" , "Soft brimless cap" , "Soldier's cap" , "felt on one's head" .)