Nick’s not a companion (5)
I believe the answer is:
notch
'nick's' is the definition.
(synonyms)
'not a companion' is the wordplay.
'a companion' becomes 'ch' (I've seen this in other clues).
'not'+'ch'='NOTCH'
(Other definitions for notch that I've seen before include "Indentation - step up" , "Secure" , "Indentation, nick" , "Nick - score" , "Dent" .)