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