Arp, for one (7)
I believe the answer is:
'arp for one' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)