Crossword Genius

It's been used to make sauce for aeons by fermentation (7)

Ross

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" .)

I've seen this clue in The Guardian.
Want a hint initially instead of a full solution? Install my app