Seed — not a bean from France — that’s primary source of sustenance (8)
I believe the answer is:
nutrient
'source of sustenance' is the definition.
(nutrients provide nutrition or sustenance)
'seed not a bean from france that's primary' is the wordplay.
'seed' becomes 'nut' (nut is a kind of seed).
'not a bean from france' becomes 'rien' ('nothing' in French).
'primary' suggests taking the first letters.
The first letter of 'thats' is 't'.
'nut'+'rien'+'t'='NUTRIENT'
(Other definitions for nutrient that I've seen before include "Something providing nourishment" , "Substance that provides nourishment, may be included in 6 Down" , "promoting growth" , "Feeding" , "vitamin maybe" .)