Macho, but only to a companion (5)
I believe the answer is:
butch
'macho' is the definition.
(synonyms)
'only to a companion' is the wordplay.
'only' becomes 'but' (but can mean only - eg 'life is but a dream').
'to a companion' becomes 'ch' (Companion of Honour).
'but'+'ch'='BUTCH'
'but' acts as a link.
(Other definitions for butch that I've seen before include "Markedly masculine" , "Macho" , "Aggressively masculine" , "Acting as a male (colloq.)" , "Tough" .)