Potato (4)
'potato' is the definition.
(spud can informally mean potato)
This is the entire clue.
(Other definitions for spud that I've seen before include "Potato (informal)" , "The humble colloquial potato" , "Potato (colloq.)" , "Commoner's King Edward?" , "King Edward, say (slang)" .)
(Another definition for masher that I've seen is " Kitchen tool for potatoes etc".)