Aquarium (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
tank
'aquarium' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tank that I've seen before include "Vat" , "Military machine" , "Container for liquids - military vehicle" , "Large container for liquid" , "Armoured vehicle; large container" .)