Set upon (6)
'set upon' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for assail that I've seen before include "Attack, lash out" , "Attack violently and persistently" , "storm" , "Go for" , "Criticise severely" .)
(Other definitions for attack that I've seen before include "Go on the offensive" , "Act violently against or criticise strongly" , "Launch an offensive" , "Assault, offensive" , "storm" .)