Let down, badly fed at the wrong time (7)
I believe the answer is:
deflate
'let down' is the definition.
(I've seen this before)
'badly fed at the wrong time' is the wordplay.
'badly' is an anagram indicator (letters in the wrong order).
'at the wrong time' becomes 'late' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fed' anagrammed gives 'def'.
'def'+'late'='DEFLATE'
Can you help me to learn more?
(Other definitions for deflate that I've seen before include "Release air from" , "Release gas from" , "Puncture, collapse like balloon" , "Let the wind out of, braggart or balloon" , "Let air out of, like balloon" .)