Half of Rothko’s rubbish (3)
I believe the answer is:
rot
'rubbish' is the definition.
(both can mean nonsense)
'half of rothko's' is the wordplay.
'half of' means to take half the letters.
'rothko' cut in half is 'ROT'.
(Other definitions for rot that I've seen before include "Deterioration" , "crumble" , "Break down, waste away" , "Twaddle" , "Gradual decline" .)