Racing tip from Ascot — bet on it! (8)
I believe the answer is:
aflutter
'racing' is the definition.
(a racing heart might be aflutter)
'tip from ascot bet on it' is the wordplay.
'tip from' says to take the initial letters.
'bet on it' becomes 'flutter' (to have a flutter is to put a bet on).
The first letter of 'ascot' is 'a'.
'a'+'flutter'='AFLUTTER'
(Other definitions for aflutter that I've seen before include "Agitated - excited (eg in anticipation)" , "in excited state" , "trembling" , "is rather nervous" , "Nervous, twitchy" .)