Alpine lake falling short — unknown quantity needed for stability (9)
I believe the answer is:
constancy
'stability' is the definition.
(I know that stability can be written as constancy)
'alpine lake falling short unknown quantity needed' is the wordplay.
'alpine lake' becomes 'Constance' (Lake Constance in central Europe).
'falling short' means to remove the last letter.
'unknown quantity' becomes 'y' (algebra).
'needed' is a charade indicator (letters next to each other).
'constance' with its final letter removed is 'constanc'.
'constanc'+'y'='CONSTANCY'
'for' is the link.
(Other definitions for constancy that I've seen before include "Unvarying faithfulness" , "The quality of being faithful and dependable" , "Faithfulness and dependability" , "Firmness - faithfulness" , "Lack of change" .)