Stick out for diamonds in authentic setting (8)
I believe the answer is:
protrude
'stick out' is the definition.
(I know that stick out can be written as protrude)
'for diamonds in authentic setting' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'diamonds in authentic setting' becomes 'trude' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pro'+'trude'='PROTRUDE'
(Other definitions for protrude that I've seen before include "Bulge" , "Come out" , "Stick outwards" , "Stick out, project from a surface" , "Stand out" .)