The little that sank in (3)
I believe the answer is:
bit
'the little' is the definition.
(a small bit)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for bit that I've seen before include "Small fragment" , "Binary digit; small piece" , "gripped" , "Small quantity or piece of metal for a horse" , "chip?" .)