Favourite pupil welcoming and unusually academic (6)
I believe the answer is:
pedant
'academic' is the definition.
(I've seen this in another clue)
'favourite pupil welcoming and unusually' is the wordplay.
'favourite pupil' becomes 'pet' (I've seen this before).
'welcoming' indicates putting letters inside (in sense of 'inviting in').
'unusually' is an anagram indicator.
'and' anagrammed gives 'dan'.
'pet' going around 'dan' is 'PEDANT'.
(Other definitions for pedant that I've seen before include "Know-all who insists on strict observance of rules" , "One concerned with excessive detail" , "Nitpicker" , "One is overly-concerned with detail" , "One particular" .)