Freeloader’s son, one that smells nasty (7)
I believe the answer is:
'freeloader's' is the definition.
'son one that smells nasty' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'one that smells nasty' becomes 'ponger' (to pong is to smell very bad).
(Other definitions for sponger that I've seen before include "Person who lives at another's expense" , "Freeloader" , "One cadging" , "cleaner?" , "One living at others' expense" .)