Powerful firm (6)
I believe the answer is:
strong
I believe this is a double definition.
'powerful' is the first definition.
(associated in meaning)
'firm' is the second definition.
(I know that firm can be written as strong)
(Other definitions for strong that I've seen before include "Great in power" , "Powerful; full-flavoured" , "Forceful" , "strong" , "is excelling" .)