Fish is too big for cask (3)

I believe the answer is:

tun

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'for cask' is the definition.
(tun is a kind of cask)

I don't understand the remainder of the clue.

(Other definitions for tun that I've seen before include "Large cask for liquid" , "Quite enough wine" , "Large cask or barrel" , "vessel" , "One can hold wine" .)

I've seen this clue in The Times.

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