Danger that holds queen back (6)
I believe the answer is:
threat
'danger' is the definition.
(threat can mean a danger or risk)
'that holds queen back' is the wordplay.
'holds' indicates putting letters inside.
'queen' becomes 'ER' (short for Elizabeth Regina ie Queen Elizabeth).
'back' shows that the letters should be reversed in order.
'er' written backwards gives 're'.
'that' going around '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" .)