Artist on streets breaking rules in ultimate provocation (4,5)
I believe the answer is:
last straw
'ultimate provocation' is the definition.
'last straw' can be an answer for 'provocation' (thesaurus). I also know that 'ultimate' relates to this answer.
'artist on streets breaking rules' is the wordplay.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'on' means one lot of letters go next to another.
'streets' becomes 'st st' (the abbreviation for street repeated).
'breaking' indicates putting letters inside.
'rules' becomes 'law' (I've seen this before).
'ra' after 'stst' is 'ststra'.
'ststra' inserted into 'law' is 'LAST STRAW'.
'in' acts as a link.
(Other definitions for last straw that I've seen before include "ultimate irritation" , "Start laws (anag) ? breaking point?" , "proved back-breaking" , "The final irritation that stretches your patience beyond the limit" , "Final blow" .)