Unopened surprise barrel (3)
I believe the answer is:
tun
'barrel' is the definition.
(tun is a kind of barrel)
'unopened surprise' is the wordplay.
'unopened' means to remove the first letter.
'surprise' becomes 'stun' (stunning is a kind of surprising).
'stun' with its first letter taken away is 'TUN'.
(Other definitions for tun that I've seen before include "butt" , "Container of beer" , "Cask, vat" , "Large cask or barrel" , "beer barrel" .)