Excellent fellow, it's said — should come promptly to 2? (5-5,4)

'2?' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'excellent fellow it's said should come promptly' is the wordplay.
I cannot really understand how this works, but
'excellent' could be 'firstclass' (I've seen this before) and 'firstclass' is located in the answer.
The remaining letters 'mail' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).

'to' acts as a link.