Thus her bet's wasted on filly finally (7)
I believe the answer is:
thereby
'thus' is the definition.
('thereby' can be a synonym of 'thus')
'her bet's wasted on filly finally' is the wordplay.
'wasted' is an anagram indicator.
'on' is an insertion indicator (as in clothing \'on\' a person).
'finally' indicates one should take the final letters.
The final letter of 'filly' is 'y'.
'bet' is an anagram of 'teb'.
'her' going within 'teb' is 'thereb'.
'thereb'+'y'='THEREBY'
(Other definitions for thereby that I've seen before include "As a result of that" , "about that amount" , "By that means" , "In consequence" , "For that reason" .)