Odds on a spring (3)
I believe the answer is:
spa
'spring' is the definition.
(spa can mean a mineral spring)
'odds on a' is the wordplay.
'odds' becomes 'sp' (betting abbreviation for starting price).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'sp'+'a'='SPA'
(Other definitions for spa that I've seen before include "Cheltenham or Bath?" , "Heated bath" , "Curative resort" , "Health resort at a mineral spring" , "Watering hole" .)