One right with a man (3)
I believe the answer is:
ira
'man' is the definition.
'one right with a' is the wordplay.
'one' becomes 'i' (Roman numeral).
'right' becomes 'r' (common abbreviation).
'with' is a charade indicator (letters next to each other).
'i'+'r'+'a'='IRA'
(Other definitions for ira that I've seen before include "terrorists" , "nationalists" , "Illegal organisation, initially" , "Militant republicans" , "George 13 down's brother" .)