Keep quiet about a Persian leader (4)
I believe the answer is:
shah
'persian leader' is the definition.
(I've seen this in another clue)
'keep quiet about a' is the wordplay.
'keep quiet' becomes 'shh'.
'about' means one lot of letters goes inside another.
'shh' placed around 'a' is 'SHAH'.
(Other definitions for shah that I've seen before include "Old Middle Eastern ruler" , "Title of Iranian monarch" , "Emperor of Iran" , "Former Iranian monarch" , "Old king" .)