Arp, for one (7)
I believe the answer is:
dadaist
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">arp for one</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for dadaist that I've seen before include "Artist favouring irreverence and incongruity" , "Absurdist artist" , "Member of nihilistic art movement of early 20th century" , "What Man Ray was" , "unconventional painter" .)