Thick coat finished in one second (7)
I believe the answer is:
impasto
'thick coat' is the definition.
(I've seen this before)
'finished in one second' is the wordplay.
'finished' becomes 'past'.
'in' means one lot of letters goes inside another.
'one' becomes 'i' (roman numeral).
'second' becomes 'mo' (both are short periods of time).
'i'+'mo'='imo'
'past' placed into 'imo' is 'IMPASTO'.
(Other definitions for impasto that I've seen before include "Art technique of laying paint on thickly" , "Thick-paint technique" , "Method of painting" , "Thickly-applied paint" , "Sort of paint" .)