Headmaster vacantly launched into difficult subject (6)
I believe the answer is:
thrall
'subject' is the definition.
(thesaurus)
'headmaster vacantly launched into difficult' is the wordplay.
'headmaster vacantly' becomes 'rh' (I can't justify this - if you can you should believe this answer much more).
'launched' says the letters should be written in reverse (in a down clue, letters get launched up).
'into' indicates putting letters inside.
'difficult' becomes 'tall' (similar in meaning).
'rh' written backwards gives 'hr'.
'hr' going inside 'tall' is 'THRALL'.
Can you help me to learn more?
(Other definitions for thrall that I've seen before include "Slave(ry)" , "State of being in another person''s power" , "Slave, servant, captive" , "State of subservience" , "subjection" .)