Harsh sound when something poisonous is swallowed (7)
I believe the answer is:
rasping
'harsh' is the definition.
('rasping' can be a synonym of 'harsh')
'sound when something poisonous is swallowed' is the wordplay.
'sound when' becomes 'ring' (ring is a kind of sound. I am not sure about the 'when' bit.**).
'something poisonous' becomes 'asp'.
'is swallowed' means one lot of letters goes inside another.
'ring' enclosing 'asp' is 'RASPING'.
(Other definitions for rasping that I've seen before include "Making a grating noise" , "Grating harshly" , "Harsh" .)