Phlegmatic (6)
I believe the answer is:
stolid
'phlegmatic' is the definition.
(synonyms)
This is all the clue.
(Other definitions for stolid that I've seen before include "cool" , "Not excitable, unemotional" , "'Calm and dependable, showing little emotion (6)'" , "Showing little or no interest or emotion" , "Difficult to excite, apathetic" .)