Grass you once found below river (3)
I believe the answer is:
rye
'grass' is the definition.
(I know that rye is a type of cereal)
'you once found below river' is the wordplay.
'you once' becomes 'ye' (archaic version of 'you').
'found below' means one lot of letters go next to another (in a down clue, some letters go below others).
'river' becomes 'r' (abbreviation).
'ye' put after 'r' is 'RYE'.
(Other definitions for rye that I've seen before include "Cereal used for making whiskey" , "E. Sussex town; cereal" , "Type of whisky" , "Cereal grass - Sussex Cinque Port" , "Grass; whisky" .)