One's learned to be vigilant about roaming charges (8)
I believe the answer is:
sheepdog
'one's' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
I can't explain the remainder of the clue.
(Other definitions for sheepdog that I've seen before include "Shaggy-haired canine" , "Canine trained to control flock" , "Farmer's herding helper" , "eg Border collie" , "Collie, perhaps" .)