Grain cost reduced by penny (4)
I believe the answer is:
rice
'grain' is the definition.
(rice is a kind of grain)
'cost reduced by penny' is the wordplay.
'cost' becomes 'price' (the cost or price of something).
'reduced by' indicates named letters should be taken away.
'penny' becomes 'p' (short for pence).
'price' with 'p' taken away is 'RICE'.
(Other definitions for rice that I've seen before include "Type of grain" , "Cereal grain" , "Cultivated food grass" , "Cereal grass" , "It grows in paddy fields" .)