Dismantle rear of fish-tank (3)

I believe the answer is:

tun

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If this is the answer, I'm afraid I cannot explain it.

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

I've seen this clue in the Sydney Morning Herald.

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