Sustenance teachers finally sought, eating nothing in Paris (8)
I believe the answer is:
nutrient
'sustenance' is the definition.
(I've seen this before)
'teachers finally sought eating nothing in paris' is the wordplay.
'teachers' becomes 'nut' (National Union of Teachers).
'finally' suggests the final letters.
'eating' means one lot of letters goes inside another.
'nothing in paris' becomes 'rien' ('nothing' in French).
The final letter of 'sought' is 't'.
'nut'+'t'='nutt'
'nutt' enclosing 'rien' is 'NUTRIENT'.
(Other definitions for nutrient that I've seen before include "Substance that provides nourishment, may be included in 6 Down" , "Feeding" , "healthy input" , "Substance to give energy and build tissue" , "Nourishing substance" .)