Politician follows back-up question (4)
I believe the answer is:
pump
'question' is the definition.
(pumping is a kind of questioning)
'politician follows back-up' is the wordplay.
'politician' becomes 'mp'.
'follows' means one lot of letters go next to another.
'back' shows that the letters should be reversed in order.
'up' in reverse letter order is 'pu'.
'mp' put after 'pu' is 'PUMP'.
(Other definitions for pump that I've seen before include "Question - means of inflation" , "Set foot in it" , "Device to move fluid or gas by pressure or suction" , "Device for driving air or liquid" , "Raise liquid or air" .)