Solid (5-11)
There are a few possibilities for this:
three-dimensional
'solid' is the definition.
(I know that solid can be written as firm)
This is all the clue.
(Other definitions for firm that I've seen before include "Unyielding business concern?" , "Steady sort of business organisation" , "Unyielding; company" , "Company - solid" , "Organization" .)
'solid' is the definition.
(associated in meaning)
This is all the clue.
(Other definitions for hard that I've seen before include "Difficult; rigid" , "Solid; tough" , "Firm; demanding; stern" , "to be listened to in Scotland" , "Unyielding" .)
'solid' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for three-dimensional that I've seen before include "Having length, breadth and width" , "Lifelike, real" .)