Instant act as broker to support firm regularly (9)
I believe the answer is:
immediate
'instant' is the definition.
(thesaurus)
'act as broker to support firm regularly' is the wordplay.
'act as broker' becomes 'mediate' (as in someone brokering a deal between two parties).
'to support' is a charade indicator (letters next to each other) (in a down clue, some letters hold others up).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternate letters of 'firm' are 'im'.
'mediate' after 'im' is 'IMMEDIATE'.
(Other definitions for immediate that I've seen before include "Prompt, straightaway" , "Most urgent; current" , "Involving no delay" , "Instant" , "Instantaneous" .)