Sign used by Marie Stopes (5)
I believe the answer is:
aries
'sign' is the definition.
(star sign)
'used by marie stopes' is the wordplay.
'used by' indicates a hidden word.
'ARIES' is hidden within 'marie stopes'.
(Other definitions for aries that I've seen before include "A sign from Heaven" , "Stars" , "Zodiacal sign also known as the Ram" , "Ram star sign" , "Philip -, English poet" .)