A come down, but not a disappointing one (5,7)

'a disappointing one' is the definition.
The definition suggests an adjective which matches the answer.

'a come down but not' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is located in the answer.
'down' could be 'd' and 'd' is found within the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?