Calm old representative retained by firm? Yes (9)
I believe the answer is:
composure
'calm' is the definition.
('composure' can be a synonym of 'calm')
'old representative retained by firm? yes' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'representative' becomes 'mp'.
'retained by' means one lot of letters goes inside another.
'firm?' becomes 'co' (a firm is a company).
'yes' becomes 'sure' (I've seen this in other clues).
'o'+'mp'='omp'
'omp' put into 'co' is 'compo'.
'compo'+'sure'='COMPOSURE'
(Other definitions for composure that I've seen before include "Calm state" , "Self-possession" , "Calmness and self-control" , "tranquillity" , "Calm self-control" .)