Contempt for a small cereal plant (5)
I believe the answer is:
scorn
'contempt' is the definition.
(I know that contempt can be written as scorn)
'small cereal plant' is the wordplay.
I cannot quite understand how this works, but
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is present in the answer.
'cereal' could be 'corn' (corn is a kind of cereal) and 'corn' is located in the answer.
No letters remain.
This explanation may well be incorrect...
'for a' is the link.
Can you help me to learn more?
(Other definitions for scorn that I've seen before include "Look at with disdain" , "Contumely" , "Spurn" , "Slight" , "Mocking contempt" .)