Phlegmatic as trading's beginning in firm (6)
I believe the answer is:
stolid
'phlegmatic' is the definition.
('stolid' can be a synonym of 'phlegmatic')
'trading's beginning in firm' is the wordplay.
'beginning' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'firm' becomes 'solid' ('solid' can be a synonym of 'firm'**).
The initial letter of 'tradings' is 't'.
't' put into 'solid' is 'STOLID'.
'as' is the link.
(Other definitions for stolid that I've seen before include "Calm, emotionless" , "with a stiff upper lip" , "Difficult to excite, apathetic" , "Strongly built" , "Unemotional - dense" .)