Killers opening drink a little backward (7)
I believe the answer is:
pistols
'killers' is the definition.
'opening drink a little backward' is the wordplay.
'opening' becomes 'slot' (slot is a kind of opening**).
'drink a little' becomes 'sip'.
'backward' is a reversal indicator.
'slot'+'sip'='slotsip'
'slotsip' written backwards gives 'PISTOLS'.
(Other definitions for pistols that I've seen before include "Guns" , "Handguns" , "weapons" .)