Calm place might erupt (10)
I believe the answer is:
Here is my best explanation:
'calm' is the definition.
'place might erupt' is the subsidiary indication.
'erupt' is an anagram indicator.
'placemight' with letters rearranged gives 'phlegmatic'.
(Other definitions for phlegmatic that I've seen before include "dull and lifeless", "Calm; philosophical", "Unemotional", "cool" and "Unexcitable".)