The opportunist's vessel (3)
I believe the answer is:
tun
'vessel' is the definition.
(type of vessel for beer)
'the opportunist's' is the wordplay.
'TUN' is hidden within the letters.
But, I am not sure how the hidden word is indicated.
Can you help me to learn more?
(Other definitions for tun that I've seen before include "Large cask or barrel" , "Nut found in large cask" , "vessel" , "Container of beer" , "Large cask for wine or beer" .)