A script: it’s about girl underground, mostly (7)
I believe the answer is:
italics
'a script' is the definition.
(type of script)
'it's about girl underground mostly' is the wordplay.
'about' indicates putting letters inside.
'girl underground' becomes 'alice' (as in Alice in Wonderland).
'mostly' means to remove the last letter.
'alice' with its final letter taken off is 'alic'.
'its' placed around 'alic' is 'ITALICS'.
(Other definitions for italics that I've seen before include "List CIA with letters that slant" , "Printed letters that slope" , "Type with slanting letters" , "Sloping font" , "Sloping typeface" .)