Sound of person dying inside container of poisonous gases (8)
I believe the answer is:
prattles
'gases' is the definition.
(both can mean to talk at length)
'sound of person dying inside container of poisonous' is the wordplay.
'sound of person dying' becomes 'rattle' (as in a death rattle).
'inside' means one lot of letters goes inside another.
'container of' says to hollow out the word (remove centre letters).
'poisonous' with its centre removed is 'ps'.
'rattle' going within 'ps' is 'PRATTLES'.
(Other definitions for prattles that I've seen before include "talks foolishly" , "Speaks incessantly about uninteresting subjects" .)