Attack when provided with canvas (6)
I believe the answer is:
assail
'attack' is the definition.
(I know that attack can be written as assail)
'when provided with canvas' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'provided with' means one lot of letters go next to another.
'canvas' becomes 'sail' (a sail is a piece of canvas).
'as'+'sail'='ASSAIL'
(Other definitions for assail that I've seen before include "Molest or harass" , "Attack, lash out" , "Aggressively attack" , "Trouble" , "Criticise severely" .)