Leader in tournament's arrested (4)
I believe the answer is:
tsar
'leader' is the definition.
(type of leader)
'in tournament's arrested' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TSAR' can be found hidden inside 'tournaments arrested'.
(Other definitions for tsar that I've seen before include "A former Russian ruler - rats to him" , "Rats (anag)" , "Nicholas II, say" , "Old leader" , "Formal rulings on Islamic law; was aft (anag.)" .)