Stretcher brought when fashion designer clutches muscle (7)
I believe the answer is:
dilator
'stretcher' is the definition.
(something which stretches or dilates an opening)
'fashion designer clutches muscle' is the wordplay.
'fashion designer' becomes 'dior' (Christian Dior).
'clutches' is an insertion indicator.
'muscle' becomes 'lat' (short for latissimus muscle).
'dior' enclosing 'lat' is 'DILATOR'.
'brought when' is the link.
(Other definitions for dilator that I've seen before include "Muscle such as that controlling eye's pupil" , "Blood pressure drug" , "Expanding, I do" .)