Lad in charge of sound waves (5)
I believe the answer is:
sonic
'of sound waves' is the definition.
(sonic means relating to sound)
'lad in charge' is the wordplay.
'lad' becomes 'son' (lad can informally mean a young boy).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'son'+'ic'='SONIC'
(Other definitions for sonic that I've seen before include "Travelling at Mach One" , "Boom of jet aircraft" , "seven hundred and forty three mph!" , "Involving sound waves, say a boom" , "Relating to the speed of sound" .)