Potato (4)
'potato' is the definition.
(spud can informally mean potato)
This is all the 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".)
(Other definitions for sweet that I've seen before include "Confection" , "Kind" , "bon-bon" , "Toffee, e.g." , "Tart, perhaps - or not tart" .)