Old Persian ruler takes in four for seven days of mourning (6)
I believe the answer is:
shivah
'days of mourning' is the definition.
The definition suggests a singular noun which matches the answer.
'old persian ruler takes in four for seven' is the wordplay.
I cannot really see how this works, but
'four' could be 'iv' (Roman numerals) and 'iv' is found within the answer.
'ruler' could be 'shah' (shah is a kind of ruler) and 'shah' is found within the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for shivah that I've seen before include "Period of mourning in Judaism" , "Jewish period of mourning" .)