It's been used to make sauce for aeons by fermentation (7)
I believe the answer is:
soybean
'it's been used to make sauce' is the definition.
(as in soy sauce)
'aeons by fermentation' is the wordplay.
'fermentation' is an anagram indicator.
'aeons'+'by'='aeonsby'
'aeonsby' with letters rearranged gives 'SOYBEAN'.
'for' is the link.
(Other definitions for soybean that I've seen before include "Vegetable source of protein" , "American vegetable" .)