Solid business (4)
I believe the answer is:
firm
This clue is a double definition.
'solid' is the first definition.
(I know that solid can be written as firm)
'business' is the second definition.
(I know that firm is a type of business)
(Other definitions for firm that I've seen before include "Organization" , "Steady and resolute" , "Unyielding; company" , "Compact - company" , "Hard" .)