Mechanical plunger (6)
I believe the answer is:
piston
'mechanical plunger' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for piston that I've seen before include "Mechanical device that has a thrusting motion" , "Valve that's instrumental" , "Block sliding in hollow cylinder in machinery etc" , "Points about driving rod" , "Driving rod in engine" .)