Son, unenthusiastic about hard tutor (6)
I believe the answer is:
school
'tutor' is the definition.
(thesaurus)
'son unenthusiastic about hard' is the wordplay.
'son' becomes 's'.
'unenthusiastic' becomes 'cool' (I've seen this before).
'about' indicates putting letters inside.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'cool' going around 'h' is 'chool'.
's'+'chool'='SCHOOL'
(Other definitions for school that I've seen before include "Train - group of porpoises" , "Group of sea creatures" , "Teach - fishes swimming" , "Educational institution" , "Academy; group of fish" .)