Sermon, eg lecture, revealing omission (7)
I believe the answer is:
neglect
'omission' is the definition.
(thesaurus)
'sermon eg lecture revealing' is the wordplay.
'revealing' indicates the answer is hidden within the clue.
'NEGLECT' can be found hidden inside 'sermon eg lecture'.
(Other definitions for neglect that I've seen before include "Insufficiently attend to" , "Lack of proper care" , "Let slide" , "slight" , "Heedlessness" .)