In the company of a financier, mostly retired (5)
I believe the answer is:
among
'in the company of' is the definition.
('among' means 'in the company of' or 'with')
'a financier mostly retired' is the wordplay.
'financier' becomes 'gnome' (informal term for a financier).
'mostly' means to remove the last letter.
'retired' says the letters should be written in reverse.
'gnome' with its last letter taken away is 'gnom'.
'gnom' in reverse letter order is 'mong'.
'a'+'mong'='AMONG'
(Other definitions for among that I've seen before include "when in company" , "No mag (anag.)" , "In the midst of (others)" , "Surrounded by" , "Within" .)