Shocking treatment following heartless email, just lost millions of voters (9)
I believe the answer is:
electoral
'of voters' is the definition.
(relating to electors)
'shocking treatment following heartless email just lost millions' is the wordplay.
'shocking treatment' becomes 'ect' (electro-convulsive therapy).
'following' is a charade indicator (letters next to each other).
'heartless' suggests removing the centre.
'just lost millions' becomes 'oral' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'email' with its centre removed is 'el'.
'ect' put after 'el' is 'elect'.
'elect'+'oral'='ELECTORAL'
(Other definitions for electoral that I've seen before include "Like sort of college" , "Relating to polls" , "concerning voters" , "List of voters" , "of constituents" .)