Solid organization (4)
I believe the answer is:
firm
I believe this is a double definition.
'solid' is the first definition.
(I've seen this before)
'organization' is the second definition.
(firm is a kind of organization)
(Other definitions for firm that I've seen before include "Resolved and steady" , "House" , "Organization" , "Strong - business" , "unlikely to collapse" .)