Speculate about Queen's headgear (5)
I believe the answer is:
beret
'headgear' is the definition.
(beret is a kind of headgear)
'speculate about queen's' is the wordplay.
'speculate' becomes 'bet' (I've seen this in other clues).
'about' means one lot of letters goes inside another.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'bet' enclosing 'er' is 'BERET'.
(Other definitions for beret that I've seen before include "Soft peak-less kind of cap" , "Soft round cap without a peak" , "Round flattish cap" , "Kind of floppy cap with no peak" , "cap in France?" .)