Betting on a spring (3)
I believe the answer is:
spa
'spring' is the definition.
(spa can mean a mineral spring)
'betting on a' is the wordplay.
'betting' becomes 'sp'.
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'sp'+'a'='SPA'
(Other definitions for spa that I've seen before include "Health resort at a mineral spring" , "here for the good of his health?" , "Health and beauty treatment establishment" , "E.g. Harrogate" , "Water-cure town" .)