Potato, informally (4)
I believe the answer is:
spud
'potato informally' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for spud that I've seen before include "Spade; potato (slang)" , "Charlotte perhaps" , "The common potato" , "Commoner's King Edward?" , "One might have an eye" .)