The penetrative part? (3)
I believe the answer is:
bit
'part?' is the definition.
(bit is a kind of part)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for bit that I've seen before include "a curb?" , "chip?" , "which is between one's teeth?" , "Small portion - part of drill" , "Mouthpiece of brindle or tip of drilling tool" .)