The little beast turns into a little angel on her entering (6)
I believe the answer is:
cherub
'the little beast turns into a little angel' is the definition.
I can't tell whether this defines the answer.
'on her entering' is the wordplay.
'on' means one lot of letters goes inside another (as in clothing \'on\' a person).
'entering' becomes 'cub' (I am not sure about this - if you are sure you should believe this answer much more).
'her' going inside 'cub' is 'CHERUB'.
Can you help me to learn more?
(Other definitions for cherub that I've seen before include "A heavenly baby" , "A chubby infant with wings" , "Angel shown as a winged child" , "Member of heavenly group" , "innocent young thing" .)