Check the key in as a rule (5)
I believe the answer is:
'as a rule' is the definition.
(I've seen this before)
'check the key in' is the wordplay.
'check' becomes 'rein' (both can mean to control or limit).
'the key' becomes 'g' (musical key).
'in' indicates putting letters inside.
'rein' enclosing 'g' is 'REIGN'.
(Other definitions for reign that I've seen before include "time in power" , "Be champion" , "Rule as king or queen" , "Period during which someone is dominant" , "Prevail" .)