Crazy Merton tutor (6)
I believe the answer is:
mentor
'tutor' is the definition.
(thesaurus)
'crazy merton' is the wordplay.
'crazy' indicates anagramming the letters.
'merton' is an anagram of 'MENTOR'.
(Other definitions for mentor that I've seen before include "Wise and experienced counsellor" , "Experienced adviser" , "Personal guide" , "Somebody advising" , "A wise and experienced teacher" .)