Bred preposterous pony finally for horse race (5)
I believe the answer is:
derby
'for horse race' is the definition.
(I've seen this in another clue)
'bred preposterous pony finally' is the wordplay.
'preposterous' is an anagram indicator.
'finally' suggests the final letters.
The last letter of 'pony' is 'y'.
'bred' anagrammed gives 'derb'.
'derb'+'y'='DERBY'
(Other definitions for derby that I've seen before include "Commotion" , "Race run at Epsom" , "Cathedral city on the River Derwent" , "Gracefully thin" , "20 horse 12" .)