Heavy cloth for sails (6)
I believe the answer is:
canvas
'heavy cloth for sails' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for canvas that I've seen before include "Intent, say" , "Sail material" , "Fabric for sail, tent and painting" , "Fabric for sail, tent and picture" , "Heavy fabric used by politicians?" .)