Indicator in support of hard school (6)
I believe the answer is:
harrow
'school' is the definition.
(famous UK public school)
'indicator in support of hard' is the wordplay.
'indicator' becomes 'arrow' (I've seen this before).
'in support of' says to put letters next to each other (in a down clue, some letters hold others up).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'arrow' after 'h' is 'HARROW'.
(Other definitions for harrow that I've seen before include "Agricultural implement - cause pain to (feelings)" , "Till" , "Torment (at public school?)" , "Farm implement with spikes" , "Distress greatly - public school" .)