Silly fellow lacking wit (3)
I believe the answer is:
'silly fellow' is the definition.
(I've seen this in another clue)
I don't understand the rest of the clue.
Can you help me to learn more?
(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" .)