Realised it had a novel sound (4)
I believe the answer is:
knew
'realised it' is the definition.
I can't judge whether this definition defines the answer.
'a novel sound' is the wordplay.
'a novel' becomes 'new' (I've seen this before).
'sound' indicates a 'sounds like' (homophone) clue.
'new' sounds like 'KNEW'.
'had' acts as a link.
Can you help me to learn more?
(Other definitions for knew that I've seen before include "Recognised" , "Was familiar with" , "Acknowledged" , "Was cognisant or aware of" , "Distinguished" .)