Disguised in company - go outside idiot! (9)
I believe the answer is:
incognito
'disguised' is the definition.
(thesaurus)
'in company go outside idiot' is the wordplay.
'company' becomes 'co' (abbreviation).
'outside' indicates putting letters inside.
'idiot' becomes 'nit' (I've seen this before).
'go' enclosing 'nit' is 'gnito'.
'in'+'co'+'gnito'='INCOGNITO'
(Other definitions for incognito that I've seen before include "With identity hidden" , "as disguised person appears?" , "In disguise" , "Covertly" , "appear unidentifiable" .)