Mother and son have hot potatoes (4)
I believe the answer is:
mash
'potatoes' is the definition.
(mashed potato)
'mother and son have hot' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'and' is a charade indicator (letters next to each other).
'son' becomes 's' (genealogical abbreviation for son).
'have' is a charade indicator (letters next to each other).
'hot' becomes 'h' (abbreviation, eg on taps).
'ma'+'s'+'h'='MASH'
(Other definitions for mash that I've seen before include "Bangers' frequent partner?" , "Soft, pulpy mass" , "Food for horses" , "Pured potato" , "Sham sort of pulp" .)