Check soundness of the rule (4)
I believe the answer is:
rein
This clue might be a double definition.
'check soundness' is the first definition.
'rein' can be an answer for 'check' (both can mean to control or limit). I am not certain of the 'soundness' bit.
'rule' is the second definition.
(I know that rule can be written as rein)
'of the' is the link.
Can you help me to learn more?
(Other definitions for rein that I've seen before include "Part of tack" , "Controlling strap" , "Restriction" , "Free ... is carte blanche" , "Harness item" .)