Impassive team-leader in firm (6)
I believe the answer is:
stolid
'impassive' is the definition.
(thesaurus)
'team-leader in firm' is the wordplay.
'leader' says to take the initial letters.
'in' indicates putting letters inside.
'firm' becomes 'solid' ('solid' can be a synonym of 'firm'**).
The initial letter of 'team' is 't'.
't' put into 'solid' is 'STOLID'.
(Other definitions for stolid that I've seen before include "Not excitable or showing emotion" , "Difficult to excite, apathetic" , "Not excitable or emotional" , "without much emotion" , "Hard to excite" .)