What overzealous Remainers do to expose support (7)
I believe the answer is:
outstay
'what overzealous remainers do' is the definition.
'expose support' is the wordplay.
'expose' becomes 'out' (outing is a kind of exposing).
'support' becomes 'stay' (stay is a kind of support**).
'out'+'stay'='OUTSTAY'
'to' is the link.
(Other definitions for outstay that I've seen before include "remain too long" , "Remain beyond" , "abuse welcome" , "Last longer than (another competitor)" , "Remain longer than" .)