The part that sank in (3)
I believe the answer is:
bit
'the part' is the definition.
(both mean a piece of something)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for bit that I've seen before include "Binary digit; small piece" , "Small piece; boring tool" , "something boring" , "Unspoken" , "gripped" .)