Interrogate trainer (4)
I believe the answer is:
pump
This could be a double definition.
'interrogate' is the first definition.
(thesaurus)
'trainer' is the second definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I don't see?
Can you help me to learn more?
(Other definitions for pump that I've seen before include "Device for driving air or liquid" , "The heart is one" , "Question - means of inflation" , "Machine for moving liquids or gases" , "Device to move fluid or gas by pressure or suction" .)