Women's quarters in which are Muslims? (5)
I believe the answer is:
harem
'women's quarters' is the definition.
(I've seen this before)
'in which are muslims?' is the wordplay.
'in' indicates a hidden word.
'HAREM' is hidden within 'which are muslims'.
(Other definitions for harem that I've seen before include "House of concubines" , "Female living quarters in a Muslim household" , "Collection of wives, concubines and female servants" , "Group of wives" , "Her ma (anag)" .)