Drain liquid that's rising in spring (3)
I believe the answer is:
sap
'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" .)