That chap’s sister initially makes sibilant noise (4)
I believe the answer is:
hiss
'noise' is the definition.
(hiss is a kind of noise)
'that chap's sister' is the wordplay.
'that chap's' becomes 'his' (possessive pronoun for a male or 'chap').
'sister' becomes 's' (this might be a standard abbreviation which I don't know about).
'his'+'s'='HISS'
'initially makes sibilant' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for hiss that I've seen before include "Make sibilant sound like snake" , "Make sibilant sound of disapproval as at villain" , "Wordless sound of disapproval" , "Audio interference" , "Urgent whisper" .)