Cereal plant, one unknown in West (5)
I believe the answer is:
maize
'cereal plant' is the definition.
(I've seen this before)
'one unknown in west' is the wordplay.
'one' becomes 'i' (Roman numeral).
'unknown' becomes 'z' (algebra).
'in' is an insertion indicator.
'west' becomes 'mae' (I've seen this before).
'i'+'z'='iz'
'iz' going within 'mae' is 'MAIZE'.
(Other definitions for maize that I've seen before include "Corncob plant" , "Corn-cob cereal" , "Type of corn" , "- maybe staple" , "one could make a meal of it" .)