Base vote on case for independent withdrawal (4)
I believe the answer is:
exit
'withdrawal' is the definition.
(I've seen this before)
'base vote on case for independent' is the wordplay.
'base' becomes 'e' (Euler's number - base of natural logarithms).
'vote' becomes 'x' (how votes are marked).
'on' says to put letters next to each other.
'case for' means to remove the middle letters (the letters which encase the others are used).
'independent' with its centre removed is 'it'.
'e'+'x'+'it'='EXIT'
(Other definitions for exit that I've seen before include "Outlet" , "Relinquish the lead" , "Move out" , "Road leaving motorway" , "walk away" .)