Danger that holds queen back (6)
I believe the answer is:
'danger' is the definition.
(threat can mean a danger or risk)
'that holds queen back' is the wordplay.
'holds' means one lot of letters goes inside another.
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'back' shows that the letters should be reversed in order.
'er' back-to-front is 're'.
'that' enclosing 're' is 'THREAT'.
(Other definitions for threat that I've seen before include "The rat became a menace" , "Potential source of danger" , "Menace of impending evil" , "Menacing words" , "An intimidating warning" .)