Improves English manuscript, including the last bit (6)
I believe the answer is:
emends
'improves' is the definition.
(emending is a kind of improving)
'english manuscript including the last bit' is the wordplay.
'english' becomes 'E' (abbreviation as in OED).
'manuscript' becomes 'ms' (abbreviation for manuscript).
'including' is an insertion indicator.
'the last bit' becomes 'end' (I've seen this in another clue).
'ms' enclosing 'end' is 'mends'.
'e'+'mends'='EMENDS'
(Other definitions for emends that I've seen before include "Improves,corrects" , "Alters, corrects" , "Send me (anag.); corrects" , "Revises" , "Alters written material to remove errors" .)