Odds on racehorse are glittering (7)
I believe the answer is:
sparkle
'are glittering' is the definition.
(thesaurus)
'odds on racehorse' is the wordplay.
'odds' becomes 'sp' (betting abbreviation for starting price).
'on' is a charade indicator (letters next to each other).
'racehorse' becomes 'arkle' (I've seen this before).
'sp'+'arkle'='SPARKLE'
(Other definitions for sparkle that I've seen before include "Gleam" , "Glisten" , "Flash brightly" , "Glitter, effervesce" , "Shine (wittily)" .)