Swallow, as a joke is heard (6)
I believe the answer is:
ingest
'swallow' is the definition.
(I've seen this before)
'as a joke is heard' is the wordplay.
'as a joke' becomes 'injest' (in jest).
'is heard' shows a homophone (sound like).
'injest' sounds like 'INGEST'.
(Other definitions for ingest that I've seen before include "Take (food) into the body" , "Eat - gin set (anag)" , "Take in with tinges" , "Absorb mentally" , "Scoff" .)