Demand heroin sister’s hiding (6)
I believe the answer is:
insist
'demand' is the definition.
(thesaurus)
'heroin sister's hiding' is the wordplay.
'hiding' indicates a hidden word.
'INSIST' can be found hidden inside 'heroin sisters'.
(Other definitions for insist that I've seen before include "Maintain firmly" , "Persist in pressing" , "Be firm in demands" , "Urge" , "Press" .)