Reliable company (4)
I believe the answer is:
firm
I believe this clue is a double definition.
'reliable' is the first definition.
(thesaurus)
'company' is the second definition.
(firm can mean a company or business)
(Other definitions for firm that I've seen before include "decided" , "'Solid, unyielding (4)'" , "Steady sort of business organisation" , "Tight; company" , "Unflinching" .)