Outstanding work from substitute (6)
I believe the answer is:
relief
This clue is a double definition.
'outstanding work' is the first definition.
(I've seen this before)
'substitute' is the second definition.
(synonyms)
'from' is the link.
(Other definitions for relief that I've seen before include "Alleviation; (financial) assistance" , "Lightening up of something" , "Person replacing another on duty" , "- - of Mafeking" , "Alleviation or support" .)