Crossword Genius

Drain liquid that's rising in spring (3)

Ross

I believe the answer is:

sap

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'drain' is the definition.
(sapping is a kind of draining)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for sap that I've seen before include "Plant's fluid" , "trench used in siege" , "Imbecile" , "The juice in trees" , "Charlie" .)

I've seen this clue in The Guardian.
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