Cost puts pressure on grain (5)
I believe the answer is:
price
'cost' is the definition.
(the cost or price of something)
'pressure on grain' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'grain' becomes 'rice' (rice is a kind of grain).
'p'+'rice'='PRICE'
'puts' acts as a link.
(Other definitions for price that I've seen before include "Cost of purchase" , "Amount for which a thing is sold" , "Fee" , "Listed cost" , "Value - figure" .)