Expert lecturer welcoming the writer back (5)
I believe the answer is:
demon
'expert' is the definition.
(I've seen this before)
'lecturer welcoming the writer back' is the wordplay.
'lecturer' becomes 'don' (I've seen this before).
'welcoming' means one lot of letters goes inside another (in sense of 'inviting in').
'the writer' becomes 'me' (the person who is writing these clues).
'back' says the letters should be written in reverse.
'me' written backwards gives 'em'.
'don' placed around 'em' is 'DEMON'.
(Other definitions for demon that I've seen before include "Evil spirit, devil" , "Fanatic; devil" , "ace" , "Bad spirit" , "beast" .)