Money? Please be quiet! (4)
I believe the answer is:
dosh
'money?' is the definition.
(I've seen this in other clues)
'please be quiet' is the wordplay.
'please' becomes 'do' (I've seen this in other clues).
'be quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'do'+'sh'='DOSH'
(Other definitions for dosh that I've seen before include "Money (sl.)" , "Lolly (slang)" , "cash" , "Moolah (slang)" , "Spondulicks" .)