It fills drains (4)

I believe the answer is:

rain

I'm a little stuck... Click here to teach me more about this clue!

'it fills' is the definition.
The definition suggests a singular noun which matches the answer.

'drains' is the wordplay.
'RAIN' is hidden within the letters.
But, I'm uncertain how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for rain that I've seen before include "Fall in large quantities" , "Cats and dogs?" , "eg A shower" , "Storm" , "Precipitation" .)

I've seen this clue in The Daily Mail.

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