First of the girls with simply tailored tunic (7)
I believe the answer is:
gymslip
'tunic' is the definition.
(I know that gymslip is a type of tunic)
'first of the girls with simply tailored' is the wordplay.
'first of' suggests taking the first letters.
'with' means one lot of letters go next to another.
'tailored' is an anagram indicator (the letters are customised ).
The initial letter of 'girls' is 'g'.
'simply' anagrammed gives 'ymslip'.
'g'+'ymslip'='GYMSLIP'
(Other definitions for gymslip that I've seen before include "Tunic worn by schoolgirls" , "Girl's short sports dress" , "old girl's uniform" , "Pre-Lycra exercise wear" , "Former schoolgirl's wear" .)