Substantial idols may be shattered (5)
I believe the answer is:
solid
'substantial' is the definition.
(I know that substantial can be written as solid)
'idols may be shattered' is the wordplay.
'may be shattered' is an anagram indicator.
'idols' with letters rearranged gives 'SOLID'.
(Other definitions for solid that I've seen before include "Firm - three-dimensional" , "Three-dimensional shape, not hollow" , "Idols (anag.)" , "Rock-hard" , "from one getting pushed around" .)