Son having dessert — or a potato? (4)
I believe the answer is:
spud
'a potato?' is the definition.
(spud can informally mean potato)
'son having dessert' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'having' means one lot of letters go next to another.
'dessert' becomes 'pud' (short for pudding).
's'+'pud'='SPUD'
'or' acts as a link.
(Other definitions for spud that I've seen before include "King Edward, e.g." , "Tattie" , "Spade; potato (slang)" , "Weed spade" , "One could be in jacket" .)