Three-dimensional (5)
'three-dimensional' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for solid that I've seen before include "Firm - substantial" , "Idols (anag.)" , "Firmly constructed" , "Strongly made or built" , "Rock-hard" .)
(Other definitions for spatial that I've seen before include "in 3-D" , "As plait (anag.)" , "Relating to space" , "A spital (anag)" , "Space-related (T, not C)" .)