Firmness (9)
'firmness' is the definition.
(I know that stability is a type of firmness)
This is the entire clue.
(Other definitions for stability that I've seen before include "State of being firmly fixed" , "not displayed by Humpty-Dumpty" , "Balance" , "Balanced, settled state" , "State of steadiness and reliability" .)
'firmness' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for solidity that I've seen before include "Substance" , "Firm quality" , "Firmness" .)