Athwart (10)
I believe the answer is:
transverse
'athwart' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for transverse that I've seen before include "Situated across something" , "Extending across (the long axis)" , "Extending across something" , "Cross" , "go in 6 ac [CRISSCROSS]" .)