Calmness of temperament (6)
I believe the answer is:
'calmness of temperament' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for phlegm that I've seen before include "Mucus - stolidity" , "Thick mucus in throat" , "Calmness of temperament" , "Mucous secretion" , "Mucus - self-possession" .)