Inveterate conservative editor keeps running company (9)
I believe the answer is:
confirmed
'inveterate' is the definition.
(thesaurus)
'conservative editor keeps running company' is the wordplay.
'conservative' becomes 'con' (abbreviation for Conservative).
'editor' becomes 'ed' (abbreviation).
'keeps' indicates putting letters inside.
'running company' becomes 'firm' (a commercial company).
'con'+'ed'='coned'
'coned' going around 'firm' is 'CONFIRMED'.
(Other definitions for confirmed that I've seen before include "Verified" , "Stated with assurance that something was true" .)