Companion to a case is confused (9)
I believe the answer is:
associate
'companion' is the definition.
('associate' can be a synonym of 'companion')
'to a case is confused' is the wordplay.
'confused' indicates an anagram.
'to'+'a'+'case'+'is'='toacaseis'
'toacaseis' anagrammed gives 'ASSOCIATE'.
(Other definitions for associate that I've seen before include "Consort" , "Partner in work or business" , "Person one works or spends time with" , "Hobnob" , "Get together with" .)