Good, simply adjusted, schoolgirl's garment (7)
I believe the answer is:
gymslip
'schoolgirl's garment' is the definition.
'good simply adjusted' is the wordplay.
'good' becomes 'g' (abbreviation).
'adjusted' is an anagram indicator.
'simply' is an anagram of 'ymslip'.
'g'+'ymslip'='GYMSLIP'
(Other definitions for gymslip that I've seen before include "Former schoolgirl's wear" , "Girl's short sports dress" , "Pre-Lycra exercise wear" , "Schoolgirl's PE uniform" , "old girl's uniform" .)