Two lines penned by each girl (4)
I believe the answer is:
ella
'girl' is the definition.
(girl's name)
'two lines penned by each' is the wordplay.
'two lines' becomes 'll' (two abbreviations for line).
'penned by' is an insertion indicator (inserted letters are penned in).
'each' becomes 'ea' (abbreviation).
'll' going into 'ea' is 'ELLA'.
(Other definitions for ella that I've seen before include "Girl" , "Miss" , "Great late jazz singer . . . . Fitzgerald" , "Ms Fitzgerald" , "Woman" .)