Quality of sound coming from piston engine (4)
I believe the answer is:
tone
'quality of sound' is the definition.
(I've seen this before)
'coming from piston engine' is the wordplay.
'coming from' says the answer is hidden in the clue.
'TONE' can be found hidden inside 'piston engine'.
(Other definitions for tone that I've seen before include "Pitch of a voice" , "Pitch (sound)" , "Vocal sound of note" , "Note as in voice" , "Note; note (anag.)" .)