Train — or part of one (5)
I believe the answer is:
coach
'train' is the definition.
(I know that train can be written as coach)
'part of one' is the wordplay.
I cannot quite see how this works, but
'of' could be 'o' and 'o' is present in the answer.
'one' could be 'a' (a thing is one thing) and 'a' is found within the answer.
This explanation may well be incorrect...
'or' is the link.
Can you help me to learn more?
(Other definitions for coach that I've seen before include "Trainer - vehicle" , "Bus, instructor" , "Railway carriage or sports instructor" , "Bus-like vehicle" , "Instruct" .)