Objection to a companion being macho! (5)
I believe the answer is:
butch
'being macho' is the definition.
('butch' can be a synonym of 'macho')
'objection to a companion' is the wordplay.
'objection' becomes 'but' (as in 'no ifs, no buts').
'to a companion' becomes 'ch' (Companion of Honour).
'but'+'ch'='BUTCH'
(Other definitions for butch that I've seen before include "Aggressively masculine" , "Tough" , "Acting as a male (colloq.)" , "Markedly masculine" , "Macho" .)