Calmness of temperament (6)
I believe the answer is:
phlegm
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">calmness of temperament</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for phlegm that I've seen before include "Mucus - stolidity" , "Thick mucus in throat" , "Calmness of temperament" , "Mucous secretion" , "Mucus - self-possession" .)