Additional way in or right of way (9)
I believe the answer is:
accessory
'additional' is the definition.
(I've seen this before)
'way in or right of way' is the wordplay.
'way in' becomes 'access' (I've seen this before).
'right of way' becomes 'y' (I am not sure about this - if you are sure you should believe this answer much more).
'access'+'or'+'y'='ACCESSORY'
Can you help me to learn more?
(Other definitions for accessory that I've seen before include "One assisting a crime" , "Adjunct" , "Supplementary item" , "additional" , "Confederate" .)