Mentioned Brian Boru, for instance, would be travelling cross-country (6)
I believe the answer is:
hiking
'travelling cross-country' is the definition.
The answer and definition can be both to do with motion as well as being present participle verbs.
Maybe they are linked in a way I don't understand?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for hiking that I've seen before include "Walking some way" , "Rambling" , "Raising" , "On a trek" , "Tramping" .)