Reject overturned ballot with victory for leader (4)
I believe the answer is:
veto
'reject overturned ballot' is the definition.
'veto' can be an answer for 'reject' (thesaurus). I'm not sure about the 'overturned ballot' bit.
'with victory for leader' is the wordplay.
'with' means one lot of letters go next to another.
'victory' becomes 've' (VE is short for Victory in Europe).
'for leader' becomes 'to' (I can't justify this - if you can you should believe this answer much more).
've' put next to 'to' is 'VETO'.
Can you help me to learn more?
(Other definitions for veto that I've seen before include "Vote that blocks a decision" , "Red light" , "Ban; refuse to allow" , "The right to forbid an action" , "Reject - vote (anag)" .)