Valet shows where cash is delivered during prohibition (6)
I believe the answer is:
batman
'valet' is the definition.
(I've seen this before)
'where cash is delivered during prohibition' is the wordplay.
'where cash is' becomes 'ATM' (automatic teller machine).
'delivered during' indicates putting letters inside.
'prohibition' becomes 'ban' (ban is a kind of prohibition).
'atm' going inside 'ban' is 'BATMAN'.
'shows' acts as a link.
(Other definitions for batman that I've seen before include "Caped crusader of Gotham City, cartoon or cinema" , "valet" , "Bantam assuming role of masked crusader" , "Cartoon hero of Gotham City" , "Cartoon hero or officer's servant" .)