High-powered firm (6)
I believe the answer is:
strong
I believe this clue is a double definition.
'high-powered' is the first definition.
(thesaurus)
'firm' is the second definition.
('strong' can be a synonym of 'firm')
(Other definitions for strong that I've seen before include "Decisive - spicy - bright - tough" , "Physically powerful - feel 1 Across" , "Not easily broken" , "is excelling" , "Powerful; full-flavoured" .)