I am finished with old paint (7)
I believe the answer is:
impasto
'paint' is the definition.
(impasto is a type of paint)
'i am finished with old' is the wordplay.
'i am' becomes 'I'm' (contraction).
'finished with' becomes 'past' (past can mean forgotten or finished).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'im'+'past'+'o'='IMPASTO'
(Other definitions for impasto that I've seen before include "Thick-paint technique" , "Sort of paint" , "Art technique of laying paint on thickly" , "Method of painting" , "Technique of laying on paint thickly so that it stands out from a surface" .)