Was familiar with the modern sound (4)
I believe the answer is:
knew
'was familiar with' is the definition.
(I've seen this before)
'the modern sound' is the wordplay.
'the modern' becomes 'new' (I've seen this before).
'sound' shows a homophone (sound like).
'new' sounds like 'KNEW'.
(Other definitions for knew that I've seen before include "Was aware of the fact" , "Distinguished" , "Was cognizant of" , "Was familiar with" , "Recognised" .)