Knew it after expelling the new boy
I believe the answer is:
kit
'boy' is the definition.
'knew it after' is the wordplay.
'knew' becomes 'k' (this might be a standard abbreviation which I don't know about).
'after' means one lot of letters go next to another.
'k'+'it'='KIT'
'expelling the new' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for kit that I've seen before include "Football top, shorts, stockings and boots?" , "Equipment, clothing" , "Old song 'Pack up your troubles in your old . . . bag'" , "Necessary equipment" , "Young beaver" .)