Mess made by gunmen in US bank? Not half! (7)
I believe the answer is:
farrago
'mess' is the definition.
(I have seen 'A mess' mean 'farrago' so perhaps 'mess' could also mean 'farrago')
'gunmen in us bank? not half' is the wordplay.
'gunmen' becomes 'ra'.
'in' indicates putting letters inside.
'us bank?' becomes 'wellsfargo'.
'not half' suggests halving the letters.
'wellsfargo' halved is 'fargo'.
'ra' put within 'fargo' is 'FARRAGO'.
'made by' is the link.
(Other definitions for farrago that I've seen before include "mess!" , "Mish-mash" , "A hotchpotch, perhaps of nonsense" , "mix-up" , "Disorderly mixture" .)