Flat indeed! (8)
I believe the answer is:
deflated
'flat' is the definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?
I can't explain the rest of the clue.
(Other definitions for deflated that I've seen before include "Let out air or gas from" , "Put down" , "Depressed like punctured balloon" , "Crestfallen" , "Collapsed due to release of gas" .)