Assign voting to oust leader (5)
I believe the answer is:
allot
'assign' is the definition.
(I know that assign can be written as allot)
'voting to oust leader' is the wordplay.
'voting' becomes 'ballot' (synonyms).
'to oust leader' means to remove the first letter (I've seen 'ousting leader of' mean this).
'ballot' with its first letter removed is 'ALLOT'.
(Other definitions for allot that I've seen before include "Dedicate" , "Apportion, share" , "Distribute in portions" , "Hand out" , "Earmark" .)