Bill in a flap (3)
I believe the answer is:
tab
This clue is a double definition.
'bill' is the first definition.
(tab is a kind of bill)
'a flap' is the second definition.
(thesaurus)
'in' is the link.
(Other definitions for tab that I've seen before include "Till bill" , "Flap; illicit drug pill (slang)" , "Tally at bar (colloq.)" , "Tag" , "The bill in a restaurant" .)