Let down when fed up and not in time (7)
I believe the answer is:
deflate
'let down' is the definition.
(I've seen this before)
'fed up and not in time' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'and' is a charade indicator (letters next to each other).
'not in time' becomes 'late' (I've seen this in another clue).
'fed' in reverse letter order is 'def'.
'def'+'late'='DEFLATE'
'when' is the link.
(Other definitions for deflate that I've seen before include "Cause to shrink or collapse" , "Let air out of, like balloon" , "Puncture, collapse like balloon" , "Release gas from" , "Let the wind out of, braggart or balloon" .)