Muslim official that is to say one bedtime prayer finally (6)
I believe the answer is:
vizier
'muslim official' is the definition.
(I've seen this before)
'that is to say one bedtime prayer finally' is the wordplay.
'that is to say' becomes 'viz' ('viz.' can be a synonym of 'that is to say'**).
'one' becomes 'i' (Roman numeral).
'finally' indicates one should take the final letters.
The last letters of 'bedtime prayer' are 'er'.
'viz'+'i'+'er'='VIZIER'
(Other definitions for vizier that I've seen before include "High officer in various Muslim states" , "Muslim councillor" , "High official in former Ottoman Empire" , "Turkish bigwig" , "Important official" .)