Quiz group of top umpires (4)
I believe the answer is:
pump
'quiz' is the definition.
(both can mean to interrogate)
'group of top umpires' is the wordplay.
'group of' indicates the answer is hidden within the clue (group of letters from the clue).
'PUMP' can be found hidden inside 'top umpires'.
(Other definitions for pump that I've seen before include "Compressing machine" , "Device for moving fluids" , "Set foot in it" , "Device for moving liquids by force or suction" , "Fluid moving device" .)