Girl's first with simply tailored tunic (7)
I believe the answer is:
gymslip
'tunic' is the definition.
(I know that gymslip is a type of tunic)
'girl's first with simply tailored' is the wordplay.
'girl's first' becomes 'g' (1st letter of 'girl').
'with' means one lot of letters go next to another.
'tailored' indicates an anagram (the letters are customised ).
'simply' is an anagram of 'ymslip'.
'g'+'ymslip'='GYMSLIP'
(Other definitions for gymslip that I've seen before include "schoolgirl attire" , "Pre-Lycra exercise wear" , "Former schoolgirl's wear" , "Tunic worn by schoolgirls" , "old girl's uniform" .)