Treacherously ignoring lecturer's greeting (4)
I believe the answer is:
ahoy
'greeting' is the definition.
(I have seen 'Nautical greeting' mean 'ahoy' so perhaps 'greeting' could also mean 'ahoy')
'treacherously ignoring lecturer's' is the wordplay.
'ignoring' suggests deleting specific letters.
'treacherously' with 'lecturers' removed is 'AHOY'.
(Other definitions for ahoy that I've seen before include "Offshore hail" , "Sailor's cry to ship" , "Nautical shout" , "Seafarer's call" , "Coastal land" .)