Two retiring setters: one Araucarian, extremely unconventional (8)
I believe the answer is:
bohemian
'unconventional' is the definition.
(similar in meaning)
'two retiring setters one araucarian extremely' is the wordplay.
I cannot quite see how this works, but
'one' could be 'an' ('a' and 'an' can mean 'one') and 'an' is found within the answer.
This explanation may well be incorrect...
(Other definitions for bohemian that I've seen before include "Unconventional artist" , "Unconventional in behaviour" , "Artistic, unconventional" , "artist who defies convention" , "Opera by Balfe 'The . . . . . . . . Girl'" .)