Waves to old servant, by the sound of it (4)
I believe the answer is:
surf
'waves' is the definition.
(surf can mean foam from waves)
'servant by the sound of it' is the wordplay.
'servant' becomes 'serf' (serf is a kind of servant).
'by the sound of it' shows a homophone (sound like).
'serf' is a homophone of 'SURF'.
'to old' 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 surf that I've seen before include "Ride waves - browse net" , "Browse through the Internet" , "Seaside froth" , "Sea foam" , "Foam created by waves" .)