Islam's regulated prayer book (6)
I believe the answer is:
missal
'prayer book' is the definition.
(missal is a kind of prayer book)
'islam's regulated' is the wordplay.
'regulated' indicates an anagram.
'islams' is an anagram of 'MISSAL'.
(Other definitions for missal that I've seen before include "Mass medium" , "Book of texts for the Catholic Mass" , "Book used in RC Mass" , "Religious book" , "A book containing prayers of the Mass" .)