Demand part for twin sister (6)
I believe the answer is:
insist
'demand' is the definition.
(thesaurus)
'part for twin sister' is the wordplay.
'part' says the answer is hidden in the clue.
'INSIST' is hidden within 'for twin sister'.
(Other definitions for insist that I've seen before include "Put one's foot down" , "Refuse to give way" , "Beg persistently and urgently" , "Be firmly demanding" , "Maintain persistently" .)