Matter of indifference? (6)
I believe the answer is:
phlegm
I believe this is a double definition.
'matter' is the first definition.
(phlegm is a kind of matter)
'indifference?' is the second definition.
(synonyms)
'of' is the link.
(Other definitions for phlegm that I've seen before include "Coolness" , "Calm temperament" , "Mucus - stolidity" , "Secretion from throat" , "Imperturbable stolidity" .)