Was aware of a novel sound (4)
I believe the answer is:
knew
'was aware' is the definition.
(I've seen this before)
'a novel sound' is the wordplay.
'a novel' becomes 'new' (I've seen this before).
'sound' indicates a 'sounds like' (homophone) clue.
'new' is a homophone of 'KNEW'.
'of' is the link.
(Other definitions for knew that I've seen before include "Had learnt; sounds like modern" , "Was conscious of" , "Understood" , "Recognised" , "Acknowledged" .)