Reversing right, declares a line for crossing (9)
I believe the answer is:
traversal
'for crossing' is the definition.
(traversal is a kind of crossing)
'reversing right declares a line' is the wordplay.
'reversing right' becomes 'tr' (I can't explain this - if you can you should give a lot more credence to this answer).
'declares' becomes 'avers' (averring is a kind of declaring).
'line' becomes 'l' (used when specifying particular lines from a poem).
'tr'+'avers'+'a'+'l'='TRAVERSAL'
Can you help me to learn more?
(Other definitions for traversal that I've seen before include "A journey across" , "Act of crossing" , "Coming across" , "A crossing (of something)" , "crossing over bridge" .)