Man supporting college principal's work of art (11)
I believe the answer is:
masterpiece
'work of art' is the definition.
(thesaurus)
'man supporting college principal's' is the wordplay.
'man' becomes 'piece' (synonyms).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'college principal' becomes 'master' (headmaster).
'piece' after 'master' is 'MASTERPIECE'.
(Other definitions for masterpiece that I've seen before include "Outstanding artwork" , "Work of outstanding artistry" , "outstanding achievement" , "Great work" , "major artwork" .)