Vessel carrying fuel stains (5)
I believe the answer is:
soils
'stains' is the definition.
('soil' can be a synonym of 'stain')
'vessel carrying fuel' is the wordplay.
'vessel' becomes 'ss'.
'carrying' means one lot of letters goes inside another.
'fuel' becomes 'oil' (I've seen this before**).
'ss' placed around 'oil' is 'SOILS'.
(Other definitions for soils that I've seen before include "Makes dirty" , "Stains" .)