Person that is offensive (6)
I believe the answer is:
sortie
'offensive' is the definition.
(both can mean a military attack)
'person that is' is the wordplay.
'person' becomes 'sort' (sort can mean a person or character).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'sort'+'ie'='SORTIE'
(Other definitions for sortie that I've seen before include "Offensive" , "Short trip; attack" , "Military foray" , "Short foray" , "Sudden attack by besieged troops" .)