Interrogate MP after backing up (4)
I believe the answer is:
pump
'interrogate' is the definition.
(thesaurus)
'mp after backing up' is the wordplay.
'after' is a charade indicator (letters next to each other).
'backing' shows that the letters should be reversed in order.
'up' reversed gives 'pu'.
'mp' put after 'pu' is 'PUMP'.
(Other definitions for pump that I've seen before include "ask a lot of questions" , "Device for moving liquids by force or suction" , "Obtain information from - a shoe" , "Cross-examine; siphon" , "Set foot in it" .)