Not supporting, initially, art nouveau, that is (4)
I believe the answer is:
anti
'not supporting' is the definition.
(I've seen this in another clue)
'initially art nouveau that is' is the wordplay.
'initially' indicates taking the first letters.
The initial letters of 'art nouveau that is' is 'ANTI'.
(Other definitions for anti that I've seen before include "Opposed to followers of Hitler" , "An opponent - opposed to party policy, etc" , "Against, informally" , "One who is against (something)" , "Adversary" .)