Drain of energy (3)

I think it could be either:

Which length description are you looking for?:

sap

'drain of energy' is the definition.
(I've seen this in another clue)

This is all the clue.

(Other definitions for sap that I've seen before include "Undermine the tree juice" , "Imbecile" , "Plant liquid or general vitality" , "Juice; energy" , "Moisture in tree or informal dunce" .)

I've seen this clue in The Times, the Universal and the Newsday.

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