Was au fait with the modern sound (4)
I believe the answer is:
knew
'was au fait with' is the definition.
I can't judge whether this definition defines the answer.
'modern sound' is the wordplay.
'modern' becomes 'new' (associated in meaning).
'sound' indicates a 'sounds like' (homophone) clue.
'new' is a homophone of 'KNEW'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for knew that I've seen before include "Acknowledged" , "Was aware of the fact" , "Was cognisant or aware of" , "Was cognizant of" , "Was familiar with" .)