Lacking a posh girl's traditional education? It's not done! (10)
I believe the answer is:
unfinished
'lacking' is the definition.
(thesaurus)
'not done' is the wordplay.
'not' becomes 'un' (I've seen this in other clues).
'done' becomes 'finished' ('finished' can be similar in meaning to 'done').
'un'+'finished'='UNFINISHED'
'a posh girl's traditional education? it's' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
(Other definitions for unfinished that I've seen before include "3 18 ... in B minor" , "Schubert's Eighth, for example" , "rough" , "Not concluded" .)