Loud fellow, this G-man? (3)
I believe the answer is:
fed
'g-man?' is the definition.
(I've seen this in another clue)
'loud fellow' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'fellow' becomes 'ed' (short for Edward).
'f'+'ed'='FED'
'this' acts as a link.
(Other definitions for fed that I've seen before include "Nourished" , "Provided courses" , "Given sustenance" , "Provided food for" , "Banking authority" .)