Silly fellow lacking wit (3)

I believe the answer is:

nit

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

I don't understand the rest of the clue.

(Other definitions for nit that I've seen before include "Cause of much head-scratching" , "Egg of parasitic insect" , "Egg of a head louse" , "Parasite's egg made of tin" , "Egg; fool" .)

I've seen this clue in the Evening Standard.

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