Extended call for peace involving a foreign ruler (4)
I believe the answer is:
shah
'foreign ruler' is the definition.
(as in the shahs of Iran)
'extended call for peace involving a' is the wordplay.
'extended call for peace' becomes 'shh' ('shhhhhhh!' is a request for silence).
'involving' means one lot of letters goes inside another.
'shh' placed around 'a' is 'SHAH'.
(Other definitions for shah that I've seen before include "Title of Iranian monarch" , "Former Iranian ruler" , "Former ruler of Persia" , "Old Middle Eastern ruler" , "Former Persian title" .)