Wedge hammered in restricts small moving part (6)
I believe the answer is:
piston
'part' is the definition.
(I have seen 'Part of pump' mean 'piston' so perhaps 'part' could also mean 'piston')
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for piston that I've seen before include "Points about driving rod" , "Transmitter of motion from camshaft to valve" , "Part of pump" , "A moving part of car engine" , "Trumpet part" .)