Impassivity (6)
I believe the answer is:
phlegm
'impassivity' is the definition.
(I know that impassivity can be written as phlegm)
This is the entire clue.
(Other definitions for phlegm that I've seen before include "'spit it out!'" , "Imperturbable stolidity" , "Thick mucus in throat" , "Coolness" , "Calmness of temperament" .)