Interrogates politician involved in matter (5)
I believe the answer is:
pumps
'interrogates' is the definition.
(I've seen this in another clue)
'politician involved in matter' is the wordplay.
'politician' becomes 'MP' (Member of Parliament).
'involved in' is an insertion indicator.
'matter' becomes 'pus' (liquid matter produced by the body).
'mp' placed into 'pus' is 'PUMPS'.
(Other definitions for pumps that I've seen before include "Shoes; inflaters" , "Moves fluid or gas by pressure or suction" , "Plimsolls" , "Dancing shoes - may be manned!" , "Cycle attachments" .)