Désirée possibly, something used principally for mashing? (4)
I believe the answer is:
spud
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for spud that I've seen before include "Starchy bit of food" , "King Edward, say (slang)" , "One could be in jacket" , "short, stocky person" , "King Edward, e.g." .)