Phlegmatic firm accepting contract's termination (6)
I believe the answer is:
stolid
'phlegmatic' is the definition.
(synonyms)
'firm accepting contract's termination' is the wordplay.
'firm' becomes 'solid' (synonyms).
'accepting' means one lot of letters goes inside another (inserted letters are accepted).
'termination' indicates one should take the final letters.
The final letter of 'contract' is 't'.
'solid' going around 't' is 'STOLID'.
(Other definitions for stolid that I've seen before include "Impassive or apathetic" , "Showing little or no interest or emotion" , "Calm, dependable, unemotional" , "Bovine" , "Emotionally insensitive" .)