Ruler protected by assistants arriving (4)
I believe the answer is:
tsar
'ruler' is the definition.
(historical term for Russian king)
'protected by assistants arriving' is the wordplay.
'protected by' indicates a hidden word.
'TSAR' can be found hidden inside 'assistants arriving'.
(Other definitions for tsar that I've seen before include "specific authority" , "Formal rulings on Islamic law; was aft (anag.)" , "Rats (anag)" , "Ruler finally executed" , "Russian leader" .)