Done in one second: paint liberally applied (7)
I believe the answer is:
impasto
'paint liberally applied' is the definition.
I can't judge whether this defines the answer.
'done in one second' is the wordplay.
'done' becomes 'past' (both can mean finished).
'in' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'second' becomes 'mo' (both are short periods of time).
'i'+'mo'='imo'
'past' going inside 'imo' is 'IMPASTO'.
Can you help me to learn more?
(Other definitions for impasto that I've seen before include "application of paint in this way" , "Technique possibly Van Gogh's" , "Technique of laying on paint thickly so that it stands out from a surface" , "Thick-paint technique" , "Thick coat required" .)