Title — Serbia’s and Russia’s leaders carry it (4)
I believe the answer is:
tsar
'carry it' is the definition.
I can't judge whether this definition defines the answer.
'title serbia's and russia's leaders' is the wordplay.
'leaders' says to take the initial letters.
The initial letters of 'title serbia and russias' is 'TSAR'.
(Other definitions for tsar that I've seen before include "Alexander and Nicholas held this title in Russia" , "Ruler finally executed" , "Potentate" , "Formal rulings on Islamic law; was aft (anag.)" , "Russian ruler (S, not Z)" .)