Bill in club, upset (3)
I believe the answer is:
tab
'bill' is the definition.
(tab is a kind of bill)
'club upset' is the wordplay.
'club' becomes 'bat' (bat is a kind of club).
'upset' shows that the letters should be reversed in order.
'bat' written backwards gives 'TAB'.
'in' acts as a link.
(Other definitions for tab that I've seen before include "Tally at bar (colloq.)" , "Marker" , "Text indent key" , "Tag" , "Running bar bill" .)