Mostly survey something to paint on (6)
I believe the answer is:
canvas
'something to paint on' is the definition.
(paintings may be painted on canvas)
'mostly survey' is the wordplay.
'mostly' means to remove the last letter.
'survey' becomes 'canvass' (canvassing is a kind of surveying).
'canvass' with its final letter taken away is 'CANVAS'.
(Other definitions for canvas that I've seen before include "Painting - sails" , "Intent, say" , "End of a racing boat; coarse cloth" , "Floor of a boxing ring" , "Fabric for sail, tent and picture" .)