Young fellow in charge of sound (5)
I believe the answer is:
sonic
'of sound' is the definition.
(I've seen this before)
'young fellow in charge' is the wordplay.
'young fellow' becomes 'son' (I've seen this in another clue).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'son'+'ic'='SONIC'
(Other definitions for sonic that I've seen before include "Producing sound" , "seven hundred and forty three mph!" , "Travelling at Mach One" , "Like mine, possibly" , "Relating to sound waves" .)