The foolishness of backing some filly (5)
I believe the answer is:
folly
'the foolishness' is the definition.
('folly' can be a synonym of 'foolishness')
'of backing some filly' is the wordplay.
'backing' suggests the final letters.
'some filly' becomes 'olly' (I can't explain this - if you can you should believe this answer much more).
The final letter of 'of' is 'f'.
'f'+'olly'='FOLLY'
Can you help me to learn more?
(Other definitions for folly that I've seen before include "Absurdity" , "'Where ignorance is bliss. 'tis . . . . . to be wise'" , "mock Gothic building" , "'Madness, stupidity (5)'" , "Foolishness" .)