Gucci to start with simply formed tunic (7)
I believe the answer is:
gymslip
'tunic' is the definition.
(gymslip is a kind of tunic)
'gucci to start with simply formed' is the wordplay.
'to start with' indicates taking the first letters.
'formed' is an anagram indicator.
The first letter of 'gucci' is 'g'.
'simply' is an anagram of 'ymslip'.
'g'+'ymslip'='GYMSLIP'
(Other definitions for gymslip that I've seen before include "old girl's uniform" , "Former schoolgirl's wear" , "Tunic worn by schoolgirls" , "Pre-Lycra exercise wear" , "Girl's short sports dress" .)