Quantity of liquid – a way to support place for fish (7)

I believe the answer is:

tankard

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But I can't explain it at all! Could you help me understand?

(Other definitions for tankard that I've seen before include "Mug-like drinking vessel" , "One-handled drinking mug" , "Beer-glass" , "Vessel tracked" , "A drinking mug with handle" .)

I've seen this clue in The Independent.

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