Silly fellow climbing a tree? (3)

I believe the answer is:

sap

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'silly' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe they are linked in a way I don't understand?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for sap that I've seen before include "Plant juice for silly ass" , "Liquid that circulates in plants" , "Watery substance in a plant" , "Fluid in plant stem" , "Drain of energy" .)

I've seen this clue in the Evening Standard.

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