Crossword Genius

One's learned to be vigilant about roaming charges (8)

Ross

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" .)

I've seen this clue in The Independent.
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