Start with this, or one could use alternate ending, on reflection (6)
I believe the answer is:
origin
'start' is the definition.
(I've seen this in another clue)
'this or one could use alternate ending on reflection' is the wordplay.
I cannot really understand how this works, but
'or' is found in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
The remaining letters 'gin' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'with' acts as a link.
Can you help me to learn more?
(Other definitions for origin that I've seen before include "spring" , "Point of creation" , "Place or point of beginning (6)" , "Beginning, starting point" , "Square one" .)