Calm place might erupt (10)
I believe the answer is:
phlegmatic
'calm' is the definition.
(I've seen this before)
'place might erupt' is the wordplay.
'erupt' indicates anagramming the letters.
'place'+'might'='placemight'
'placemight' is an anagram of 'PHLEGMATIC'.
(Other definitions for phlegmatic that I've seen before include "dull and lifeless" , "Calm; philosophical" , "Unemotional" , "cool" , "Unexcitable" .)