Let down G-man turned up dead (7)
I believe the answer is:
deflate
'let down' is the definition.
(I've seen this before)
'g-man turned up dead' is the wordplay.
'g-man' becomes 'fed' (I've seen this in another clue).
'turned up' is a reversal indicator.
'dead' becomes 'late' (similar in meaning).
'fed' in reverse letter order is 'def'.
'def'+'late'='DEFLATE'
(Other definitions for deflate that I've seen before include "Puncture, collapse like balloon" , "Release gas from" , "Take air" , "Release air from" , "Let air out of, like balloon" .)