Sound from egg on penetration (7)
I believe the answer is:
insight
'penetration' is the definition.
(I know that penetration can be written as insight)
'sound from egg on' is the wordplay.
'sound from' indicates a 'sounds like' (homophone) clue.
'egg on' becomes 'incite' (synonyms).
'incite' is a homophone of 'INSIGHT'.
(Other definitions for insight that I've seen before include "Penetrative quality" , "apprehension" , "Visible" , "famous Sunday Times team" , "ability to perceive clearly" .)