Would be a fool to weaken (3)

I believe the answer is:

sap

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

'weaken' is the definition.
(I've seen this in another clue)

I cannot understand how the remainder of the clue works.

(Other definitions for sap that I've seen before include "Fluid in plant stem" , "Undermine the tree juice" , "Charlie" , "Watery substance in plants" , "Gradually weaken (a person's strength)" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app