Regrettably, Function follows Melchester’s Race Meeting (5,5)
I believe the answer is:
royal ascot
'race meeting' is the definition.
(I've seen this before)
'regrettably function follows melchester's' is the wordplay.
'regrettably' becomes 'alas' ('alas' is said before regrettable information).
'function' becomes 'cot' (short for cotagent, a mathematical function).
'follows' means one lot of letters go next to another.
'melchester' becomes 'roy' (I can't justify this - if you can you should give a lot more credence to this answer).
'alas'+'cot'='alascot'
'alascot' after 'roy' is 'ROYAL ASCOT'.
Can you help me to learn more?
(Another definition for royal ascot that I've seen is " race meeting".)