Vote for remain primarily, providing May's last thrill (9)
I believe the answer is:
'thrill' is the definition.
'vote for remain primarily providing may's last' is the wordplay.
'vote for' becomes 'elect' (I've seen this before).
'primarily' indicates taking the first letters.
'providing' becomes 'if' (I've seen this in other clues).
'last' indicates one should take the final letters.
The initial letter of 'remain' is 'r'.
The last letter of 'may' is 'y'.
(Other definitions for electrify that I've seen before include "Fire" , "Charge with power" , "Galvanise - animate" , "Supply power to; shock, thrill" , "Excite intensely" .)