Amount canvas destroyed? (6)
I believe the answer is:
extent
'amount canvas destroyed?' is the definition.
I can't judge whether this defines the answer.
'amount canvas' is the wordplay.
'amount' becomes 'ex' (I can't justify this - if you can you should believe this answer much more).
'canvas' becomes 'tent' (I've seen this in other clues).
'ex'+'tent'='EXTENT'
(Other definitions for extent that I've seen before include "Area something covers" , "Range or scope of something" , "Degree - amount - scope" , "reach?" , "Area covered by something" .)