Substantial figure (5)
I believe the answer is:
solid
I believe this clue is a double definition.
'substantial' is the first definition.
(synonyms)
'figure' is the second definition.
(solid figure is a kind of figure)
(Other definitions for solid that I've seen before include "Concrete or reilable" , "Shape with three dimensions" , "Not hollow or liquid" , "Reliable - strong" , "Firmly constructed" .)