Flipping mug is ex-PM! (5)
I believe the answer is:
boris
'ex-pm' is the definition.
(Boris Johnson)
'flipping mug is' is the wordplay.
'flipping' shows that the letters should be reversed in order.
'mug' becomes 'rob' (to mug someone is to rob them).
'rob' in reverse letter order is 'bor'.
'bor'+'is'='BORIS'
(Other definitions for boris that I've seen before include "- Johnson, Mayor of London" , "- Pasternak, Russian author (Doctor Zhivago)" , "Russian name" , "Leading Brexiteer's given name" , "-- Godunov; -- Johnson" .)