Sky confusing extremes of modernism with fine art (9)
I believe the answer is:
Here is my best explanation:
'sky' is the definition.
(I've seen this before)
'confusing extremes of modernism with fine art' is the subsidiary indication.
'confusing' is an anagram indicator.
'extremes of' means to remove the middle letters.
'with' says to put letters next to each other.
'modernism' with its centre removed is 'mm'.
'mm' put after 'fineart' is 'fineartmm'.
'fineartmm' is an anagram of 'firmament'.
(Other definitions for firmament that I've seen before include "The celestial sphere", "Heavens, sky", "The starry heaven", "Sky (literary)", "outer space, encapsulated" and "The heavens".)