Attack or shun perhaps (6)
I believe the answer is:
onrush
'attack' is the definition.
(I know that attack can be written as onrush)
'or shun perhaps' is the wordplay.
'perhaps' indicates anagramming the letters.
'or'+'shun'='orshun'
'orshun' is an anagram of 'ONRUSH'.
(Other definitions for onrush that I've seen before include "Swarm" , "Forceful forward flow" , "Forward charge of Hurons" , "Forward dash" , "sudden swoop" .)