Crossword Genius

Water tower empty when turned over (3)

Ross

I believe the answer is:

tug

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'water tower' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for tug that I've seen before include "Haul" , "Pull a form of boat" , "Small ship for pulling" , "Pull sharply" , "Pull hard; boat" .)

I've seen this clue in The Times and The Sunday Times.
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