Restrictive order requires detailed check on small number (6)
I believe the answer is:
curfew
'restrictive order' is the definition.
'detailed check on small number' is the wordplay.
'detailed check on' becomes 'cur' (I can't justify this - if you can you should give a lot more credence to this answer).
'small number' becomes 'few'.
'cur'+'few'='CURFEW'
'requires' is the link.
(Other definitions for curfew that I've seen before include "Early bedtime enforced by military" , "when it's time to go home" , "Early closing time imposed by military" , "Restriction on being out after dark" , "Prohibition order" .)