Multiple parties? Firm came down on holding one (9)
I believe the answer is:
coalition
'multiple parties?' is the definition.
(a political coalition involves more than one party)
'firm came down on holding one' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'came down' becomes 'alit' (alight can mean to land).
'holding' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'alit'+'on'='aliton'
'aliton' going around 'i' is 'alition'.
'co'+'alition'='COALITION'
(Other definitions for coalition that I've seen before include "Temporary alliance" , "Alliance of parties" , "form of government" .)