Round piece is in an abstract style, employing optical illusion (2,3)
I believe the answer is:
op art
'optical illusion' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'round piece is in an abstract style' is the wordplay.
I cannot really see how this works, but
'round' could be 'o' (o is a circular letter) and 'o' is found in the answer.
'piece' could be 'part' (synonyms) and 'part' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'employing' acts as a link.
Can you help me to learn more?
(Other definitions for op art that I've seen before include "pictures" , "Illusory form of painting" , "Abstract visual style for the 1960s, giving the illusion of movement" , "Illusory paintings" , "Sixties movement" .)