Related Clues:

• Restriction -- Father Christmas's helpers reduced by half
• Check soundness of rule
• Queen turned over in check
• Check report of time in power
• Animal controller
• Rule out government control
• Pun, maybe
• Curb (with "in")
• Jockey's brake
• One might start 'Knock knock ...'
• Keep back, with ''in''
• Engineers regularly raised limit
• Check behaviour in Red Bulletin doing the rounds
• Strap which comes down, by the sound of it?
• One-liner, for instance