Pilot lowering an echo over sound system (6)
I believe the answer is:
stereo
'sound system' is the definition.
(stereo is a kind of sound system)
'pilot lowering an echo over' is the wordplay.
'pilot lowering an echo' becomes 'stere' (I am not sure about this - if you are sure you should believe this answer much more).
'over' becomes 'o' (cricket abbreviation).
'stere'+'o'='STEREO'
(Other definitions for stereo that I've seen before include "Of sound equipment, having two speakers" , "Description of record" , "Twin-channel sound" , "Means of amplification of sound through two or more channels" , "Sound - not mono" .)