One of those in March showing the way perhaps (12)
I believe the answer is:
demonstrator
'one' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'those in march showing the way perhaps' is the wordplay.
I cannot really understand how this works, but
'march' could be 'demo' (I've seen this before) and 'demo' is found within the answer.
'way' could be 'st' and 'st' is present in the answer.
This may be the basis of clue (or it may be nonsense).
'of' acts as a link.
Can you help me to learn more?
(Other definitions for demonstrator that I've seen before include "shower" , "Participant in a public protest" , "Rally participant" , "Teacher - salesperson - someone on a march" , "showroom car" .)