Some script — girl mostly follows it (6)
I believe the answer is:
italic
'some script' is the definition.
(type of writing or script)
'girl mostly follows it' is the wordplay.
'girl mostly' becomes 'alic' (I am not sure about this - if you are sure you should believe this answer much more).
'follows' says to put letters next to each other.
'alic' after 'it' is 'ITALIC'.
(Other definitions for italic that I've seen before include "Sloping font" , "Printing style used for emphasis" , "not exactly an upright type" , "leaning over" , "from Rome?" .)