Women’s quarters, some time back, kept by monarch (5)
I believe the answer is:
harem
'women's quarters' is the definition.
(historically, women's quarters in Muslim household)
'time back kept by monarch' is the wordplay.
'time' becomes 'era' (an era is a period of time).
'back' shows that the letters should be reversed in order.
'kept by' indicates putting letters inside.
'monarch' becomes 'HM' (abbreviation for His or Her Majesty).
'era' backwards is 'are'.
'are' going inside 'hm' is 'HAREM'.
'some' acts as a link.
(Other definitions for harem that I've seen before include "Living quarters for wives and concubines" , "House of concubines" , "many maidens!" , "Living quarters of wives in a Muslim household" , "Group of wives" .)