Crossword Genius

In a depression look for somewhere to drink (6)

Ross

I believe the answer is:

trough

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'in' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for trough that I've seen before include "basin" , "Container from which cattle or horses feed" , "Hollow; channel" , "Water manger" , "Container for watering or feeding animals" .)

I've seen this clue in The Globe and Mail.
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