Some glum as having potato dish (4)
I believe the answer is:
mash
'potato dish' is the definition.
(mashed potato)
'some glum as having' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'MASH' is hidden within 'glum as having'.
(Other definitions for mash that I've seen before include "Crush, potatoes say" , "Soft, pulpy mass" , "Food for horses" , "Sausages' traditional partner" , "Pulped potatoes" .)