Sister of Meg, Jo and Amy (4)
I believe the answer is:
beth
'sister of meg jo and amy' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for beth that I've seen before include "Hebrew letter; girl's name" , "Hebrew B" , "Girl" , "Second Hebrew letter" , "Meg, Jo, - - and Amy" .)