Professor entertaining the old dean (5)
I believe the answer is:
doyen
'dean' is the definition.
(synonyms)
'professor entertaining the old' is the wordplay.
'professor' becomes 'don' (term for a university professor).
'entertaining' indicates putting letters inside.
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'don' placed around 'ye' is 'DOYEN'.
(Other definitions for doyen that I've seen before include "Senior practitioner" , "Senior member of a professional group" , "Grand old man" , "Most valued prof, eg" , "Senior member of a profession or academy" .)