Sky confusing extremes of modernism with fine art (9)
I believe the answer is:
firmament
'sky' is the definition.
(thesaurus)
'confusing extremes of modernism with fine art' is the wordplay.
'confusing' indicates an anagram.
'extremes of' says to hollow out the word (remove centre letters).
'with' is a charade indicator (letters next to each other).
'modernism' with its centre taken out is 'mm'.
'fine'+'art'='fineart'
'mm' after 'fineart' is 'fineartmm'.
'fineartmm' is an anagram of 'FIRMAMENT'.
(Other definitions for firmament that I've seen before include "outer space, encapsulated" , "Heavens, sky" , "The celestial sphere" , "The starry heaven" , "Sky (literary)" .)