Ex-PM: alternatively, former role of Boris (5)
I believe the answer is:
mayor
'former role of boris' is the definition.
(Boris Johnson was previously Mayor of London)
'ex-pm alternatively' is the wordplay.
'ex-pm' becomes 'May' (Theresa May former prime minister).
'alternatively' becomes 'or'.
'may'+'or'='MAYOR'
(Other definitions for mayor that I've seen before include "For example, Henchard" , "Local dignitary" , "Town's ceremonial head" , "Civic official" , "First citizen of a city" .)