Disgusted over large area of oil on water (5)
I believe the answer is:
slick
'area of oil on water' is the definition.
(oil slick)
'disgusted over large' is the wordplay.
'disgusted' becomes 'sick' (sick can mean feeling disgusted).
'over' is an insertion indicator.
'large' becomes 'L' (eg in clothes sizes).
'sick' enclosing 'l' is 'SLICK'.
(Other definitions for slick that I've seen before include "it's very oily" , "Bedeck" , "Showing Polish" , "Sleek - film of oil" , "Smooth" .)