Initially coming in, also out — 'hi' or 'bye'! (4)
I believe the answer is:
ciao
'bye' is the definition.
(I've seen this in another clue)
'initially coming in also out' is the wordplay.
'initially' suggests taking the first letters.
The first letters of 'coming in also out' is 'CIAO'.
'hi or' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for ciao that I've seen before include "Italian's so long" , "Hello and goodbye to the Italians" , "Italian greeting at meeting or parting" , "Italian salutation" , "European's greeting" .)