Attack a second canvas (6)
I believe the answer is:
assail
'attack' is the definition.
('assail' can be a synonym of 'attack')
'a second canvas' is the wordplay.
'a second' becomes 'as' (I've seen this in other clues).
'canvas' becomes 'sail' ('sail' can be a synonym of 'canvas').
'as'+'sail'='ASSAIL'
(Other definitions for assail that I've seen before include "Set upon" , "Attack violently and persistently" , "Criticize" , "Maybe mug" , "Beset" .)