Threat posed by introduction of Calor gas (7)
I believe the answer is:
Here is my best explanation:
'gas' is the definition.
'threat posed by introduction of calor' is the subsidiary indication.
I cannot quite understand how this works, but
an anagram of 'threat' is 'hatter' which is present in the answer.
A single letter 'c' remains which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
(Other definitions for chatter that I've seen before include "Talk rapidly", "Talk informally", "Speak rapidly and incessantly", "Idle gossip", "Trivial conversation", "Talk incessantly", "Small talk", "Informal talk", "Talk idly" and "Prattle".)