Stable sides of supporting structure (4)
I believe the answer is:
sure
'stable' is the definition.
(thesaurus)
'sides of supporting structure' is the wordplay.
'sides of' says to hollow out the word (remove centre letters) (only the letters on the sides remain).
'supporting structure' becomes 'substructure' (substructure is a kind of supporting structure).
'substructure' with its centre taken out is 'SURE'.
(Other definitions for sure that I've seen before include "Certain to be a ruse" , "certainly in the USA" , "Convinced" , "Certain, confident" , "The old sewer" .)