Ban day of celebration supported by electorate essentially (4)
I believe the answer is:
veto
'ban' is the definition.
(I've seen this before)
'day of celebration supported by electorate essentially' is the wordplay.
'day of celebration' becomes 'VE' (VE Day, Victory in Europe Day).
'supported by' means one lot of letters go next to another (in a down clue, some letters hold up others).
'essentially' says to take the centre.
The middle of 'electorate' is 'to'.
've'+'to'='VETO'
(Other definitions for veto that I've seen before include "Power to reject legislation" , "Right to reject a proposal" , "UN rejection" , "Right of rejection" , "Authoritative prohibition" .)