Rampaging so in vain attack (8)
I believe the answer is:
invasion
'attack' is the definition.
(invasion is a kind of attack)
'rampaging so in vain' is the wordplay.
'rampaging' indicates anagramming the letters (I've seen this in other clues).
'so'+'in'+'vain'='soinvain'
'soinvain' is an anagram of 'INVASION'.
(Other definitions for invasion that I've seen before include "Entry into foreign country to conquer it" , "Entry by force by an army" , "Unwelcome encroachment on another country" , "Unwelcome intrusion" , "Encroachment by an army" .)