Bill bat backwards (3)
I believe the answer is:
tab
'bill' is the definition.
(a tab is a bill at a bar)
'bat backwards' is the wordplay.
'backwards' shows that the letters should be reversed in order.
'bat' back-to-front is 'TAB'.
(Other definitions for tab that I've seen before include "Till bill" , "Small flap - bill" , "Tally at bar (colloq.)" , "Tally of orders in bar" , "Flap; meal bill" .)