Outstanding work of art (6)
I believe the answer is:
relief
This clue is a double definition.
'outstanding work' is the first definition.
(I've seen this before)
'art' is the second definition.
(I know that relief is a type of sculpture)
'of' acts as a link.
(Other definitions for relief that I've seen before include "Easement or assuagement" , "replacement staff member" , "Alleviation, easing" , "Alleviation of pain or distress" , "Easement from pain or distress" .)