Moving part to stop in breakdown (6)
I believe the answer is:
piston
'moving part' is the definition.
'piston' can be an answer for 'part' (I have seen 'Part of pump' mean 'piston' so perhaps 'part' could also mean 'piston'). I'm unsure of the 'moving' bit.
'stop in breakdown' is the wordplay.
'breakdown' indicates an anagram.
'stop'+'in'='stopin'
'stopin' is an anagram of 'PISTON'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for piston that I've seen before include "Block sliding in hollow cylinder in machinery etc" , "Mechanical device that has a thrusting motion" , "Driving rod in engine" , "Mechanical plunger" , "Brass instrument's valve" .)