Force one masculine attitude (6)
I believe the answer is:
impose
'force' is the definition.
(to impose oneself is to force oneself)
'one masculine attitude' is the wordplay.
'one' becomes 'I' (Roman numeral).
'masculine' becomes 'm'.
'attitude' becomes 'pose' (attitude can mean a pose or stance).
'i'+'m'+'pose'='IMPOSE'
(Other definitions for impose that I've seen before include "As task, set" , "Put or lay on- say a tax" , "Foist" , "Inflict (something unpleasant)" , "Exact or levy, say tax" .)