23a Composer of the year as represented in pieces(7) V???L?I VIVALDI(?)
26a One emperor backed another's followers(5) R???A
19d In brief, part of speech is edited as suggested(7) A?V???D ADVISED?
1d Not in union - albeit, oddly, in organisation allowing it(8) ??L?????
8d It's clear oddly deficient clues irritate(6) N?T?L? NETTLE?