Chances upon a spring (3)
I believe the answer is:
spa
'spring' is the definition.
(spa can mean a mineral spring)
'chances upon a' is the wordplay.
'chances' becomes 'sp' (abbreviation for starting price in gambling).
'upon' is a charade indicator (letters next to each other).
'sp'+'a'='SPA'
(Other definitions for spa that I've seen before include "Bath - mineral water spring" , "well" , "Water-cure town" , "Bath, possibly" , "Healthy place" .)