Fred Winter trained this Irish chestnut 50-1 winner of the 1966 Grand National (5)

'fred winter trained this irish chestnut' is the definition.
The definition suggests a singular noun which matches the answer.

'50-1 winner of the 1966 grand national' is the wordplay.
I cannot quite understand how this works, but
'50' could be 'l' (Roman numeral) and 'l' is found within the answer.
'1' could be 'a' (both can mean one) and 'a' is present in the answer.
'of' could be 'o' and 'o' is found in the answer.
'grand' could be 'g' (G is short for grand as in a thousand) and 'g' is found in the answer.
'national' could be 'n' (abbreviation listed in Chambers) and 'n' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?