Dismiss after chatter source of heat (3,4)
I believe the answer is:
gas fire
'source of heat' is the definition.
(I've seen this before)
'dismiss after chatter' is the wordplay.
'dismiss' becomes 'fire' (synonyms).
'after' means one lot of letters go next to another.
'chatter' becomes 'gas' (I've seen this before).
'fire' put after 'gas' is 'GAS FIRE'.
(Other definitions for gas fire that I've seen before include "Glower" , "Domestic form of heater" , "source of heat" .)