Edifies the vacuous English knight in need of reform (10)

'edifies' is the definition.
('enlighten' can be a synonym of 'edify')

'english knight in need of reform' is the wordplay.
'english' becomes 'e' (abbreviation).
'knight' becomes 'N' (chess abbreviation).
'in need of' is a charade indicator (letters next to each other).
'reform' becomes 'lightens' (I can't explain this - if you can you should believe this answer much more).
'e'+'n'+'lightens'='ENLIGHTENS'

'the vacuous' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.

Can you help me to learn more?