Step - or part of one (5)
I believe the answer is:
tread
This clue might be a double definition.
'step' is the first definition.
(I know that step can be written as tread)
'part of one' is the second definition.
'tread' can be an answer for 'part' (I have seen 'Part of tyre ' mean 'tread' so perhaps 'part' could also mean 'tread'). I am unsure of the 'of one' bit.
'or' is the link.
Can you help me to learn more?
(Other definitions for tread that I've seen before include "Part of flight" , "Depth of grooving in a tyre" , "between the car and the road" , "Step (heavily)" , "Tyre moulding" .)