Outstanding work from substitute (6)
I believe the answer is:
relief
I believe this clue is a double definition.
'outstanding work' is the first definition.
(I've seen this before)
'substitute' is the second definition.
(I know that substitute can be written as relief)
'from' acts as a 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" .)