Part of a wheel that projects about a centimetre (3)
I believe the answer is:
cam
'part of a wheel that projects' is the definition.
'cam' can be an answer for 'part' (I have seen 'Part of engine' mean 'cam' so perhaps 'part' could also mean 'cam'). I am not sure about the rest of the definition.
'about a centimetre' is the wordplay.
'about' is an insertion indicator.
'centimetre' becomes 'cm' (synonyms).
'a' placed within 'cm' is 'CAM'.
Can you help me to learn more?
(Other definitions for cam that I've seen before include "Rotating part in machine; river" , "English river" , "Tributary of the Ouse" , "Eccentric contrivance" , "Shaft on a rotating part of machine" .)