Class that is offensive (6)
I believe the answer is:
sortie
'offensive' is the definition.
(both can mean a military attack)
'class that is' is the wordplay.
'class' becomes 'sort' (synonyms).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'sort'+'ie'='SORTIE'
(Other definitions for sortie that I've seen before include "Single military flight" , "Short foray" , "Attack launched from place of siege" , "(Siege) breakout" , "Military aircraft flight" .)