Educational institution originally training young Mitford sister (5)
I believe the answer is:
unity
'mitford sister' is the definition.
(Unity Mitford was one of the Mitford sisters)
'educational institution originally training young' is the wordplay.
'educational institution' becomes 'uni' (short for university).
'originally' indicates taking the first letters.
The initial letters of 'training young' is 'ty'.
'uni'+'ty'='UNITY'
(Other definitions for unity that I've seen before include "Accord" , "Completeness (with nothing wanting)" , "State of complete and harmonious whole" , "Mitford girl" , "State of being joined as a whole" .)