Check key's in, that's the rule (5)
I believe the answer is:
'the rule' is the definition.
(reign is a kind of rule)
'check key's in that's' is the wordplay.
'check' becomes 'rein' (both can mean to control or limit**).
'key' becomes 'g' (I've seen this in other clues).
'in that's' means one lot of letters goes inside another.
'rein' placed around 'g' is 'REIGN'.
(Other definitions for reign that I've seen before include "Rule as king or queen" , "period in power" , "Control" , "Have sovereign power" , "Sovereign's rule" .)