Clearly prof ignoring PR lessons (2,6)
I believe the answer is:
of course
'clearly' is the definition.
(thesaurus)
'prof ignoring pr lessons' is the wordplay.
'ignoring' suggests deleting specific letters.
'lessons' becomes 'course' (a course has lessons).
'prof' with 'pr' taken out is 'of'.
'of'+'course'='OF COURSE'
(Other definitions for of course that I've seen before include "Naturally, as one would expect" , "need I say" , "'Naturally, as might be expected (2,6)'" , "Without a doubt, as was to be expected" , "without question" .)