Professor emptied every lecture (6)
I believe the answer is:
preach
'lecture' is the definition.
('preach' can be a synonym of 'lecture')
'professor emptied every' is the wordplay.
'emptied' means to remove the middle letters.
'every' becomes 'each' ('each' can be a synonym of 'every').
'professor' with its centre taken out is 'pr'.
'pr'+'each'='PREACH'
(Other definitions for preach that I've seen before include "Sermonise" , "A perch (anag.)" , "Publicly proclaim" , "Give moral or religious advice" , "Give, eg, a sermon" .)