Go for a small sheet of canvas (6)
I believe the answer is:
assail
'go for' is the definition.
(thesaurus)
'a small sheet of canvas' is the wordplay.
'a small sheet of' becomes 'as' (I can't explain this - if you can you should believe this answer much more).
'canvas' becomes 'sail' ('sail' can be a synonym of 'canvas').
'as'+'sail'='ASSAIL'
Can you help me to learn more?
(Other definitions for assail that I've seen before include "Sudden attack" , "Assault, attack" , "Trouble" , "Launch an attack" , "Set upon" .)