Youthful setter maybe eating starter of mixed grill (4)
I believe the answer is:
'grill' is the definition.
(both can mean to interrogate)
'youthful setter maybe eating starter of mixed' is the wordplay.
'youthful setter maybe' becomes 'pup' (setter is a type of dog).
'eating' means one lot of letters goes inside another.
'starter of' says to take the initial letters.
The first letter of 'mixed' is 'm'.
'pup' placed around 'm' is 'PUMP'.
(Other definitions for pump that I've seen before include "Device for driving air or liquid" , "Raise liquid or air" , "Device for moving liquids by force or suction" , "Shoe - inflating device" , "Light shoe worn for dancing" .)