Preserve vessel for oil (6)
I believe the answer is:
canvas
'oil' is the definition.
(thesaurus)
'preserve vessel' is the wordplay.
'preserve' becomes 'can' (canning is a kind of preserving).
'vessel' becomes 'vas' (synonyms).
'can'+'vas'='CANVAS'
'for' acts as a link.
(Other definitions for canvas that I've seen before include "Fabric for sail, tent and painting" , "Strong coarse cloth" , "Sailcloth" , "Heavy cloth of sails, tents etc" , "Fabric for sail, tent and picture" .)