Devotee of art in revolutionary Spain (8)
I believe the answer is:
partisan
'devotee' is the definition.
(thesaurus)
'art in revolutionary spain' is the wordplay.
'in' means one lot of letters goes inside another.
'revolutionary' is an anagram indicator.
'spain' anagrammed gives 'pisan'.
'art' going within 'pisan' is 'PARTISAN'.
'of' is the link.
(Other definitions for partisan that I've seen before include "Aspirant to being biased" , "Highly biased" , "Muslim call to prayer" , "In Sparta (anag)" , "Arts pain (anag.)" .)