Extreme art set up after the middle of July (5)
I believe the answer is:
ultra
'extreme' is the definition.
(I've seen this before)
'art set up after the middle of july' is the wordplay.
'set up' says the letters should be written in reverse (in a down clue, letters go up).
'after' is a charade indicator (letters next to each other).
'the middle of' indicates the central letters.
The central letters of 'july' are 'ul'.
'art' back-to-front is 'tra'.
'tra' after 'ul' is 'ULTRA'.
(Other definitions for ultra that I've seen before include "Prefix meaning excessively or extremely" , "Prefix meaning beyond or extremely" , "Immoderate" , "Extreme - extremist" , "extraordinary" .)