Auditor's rule check (4)
I believe the answer is:
rein
'check' is the definition.
(both can mean to control or limit)
'auditor's rule' is the wordplay.
'auditor's' indicates a 'sounds like' (homophone) clue.
'rule' becomes 'reign' (reign is a kind of rule).
'reign' sounds like 'REIN'.
(Other definitions for rein that I've seen before include "Restraining lead" , "A means of control" , "Long strap to restrain" , "Horse controller" , "Curb" .)