An action that's highly offensive? (3,4)
I believe the answer is:
air raid
'an action' is the definition.
(air raid is a kind of action)
'highly offensive?' is the wordplay.
'highly' becomes 'air' (I can't justify this - if you can you should believe this answer much more).
'offensive?' becomes 'raid' (a raid is a military offensive).
'air'+'raid'='AIR-RAID'
'that's' is the link.
Can you help me to learn more?
(Other definitions for air raid that I've seen before include "Bombing attack" , "Attack by hostile aircraft" , "Bombing mission" , "In wartime attack" , "Attack from the sky" .)