Painting over one line (3)
I believe the answer is:
oil
'painting' is the definition.
(thesaurus)
'over one line' is the wordplay.
'over' means one lot of letters go next to another.
'one' becomes 'oi' (I can't explain this - if you can you should believe this answer much more).
'line' becomes 'l'.
'oi'+'l' is 'OIL'.
(Other definitions for oil that I've seen before include "Viscous substance for lubrication" , "SECOND PART OF 23" , "Type of fuel" , "eg Olive" , "Lubricate; fuel" .)