Theoretical Rothko painting? (8)
I believe the answer is:
abstract
'theoretical' is the definition.
(similar in meaning)
'rothko painting?' is the wordplay.
I cannot really see how this works, but
'painting?' could be 'act' (painting is a kind of act) and 'act' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for abstract that I've seen before include "hard to understand" , "Theoretical, not concrete" , "Remove - what isn't tangible" , "Rothko work?" , "Summary; remove" .)