Crossword Genius

The opportunist's vessel (3)

Ross

I believe the answer is:

tun

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in the Evening Standard.
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