Solid company (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)
'company' is the second definition.
(firm can mean a company or business)
(Other definitions for firm that I've seen before include "team of doctors in hospital" , "Company; steady" , "corporation?" , "securely rooted" , "Unyielding business concern?" .)