Crossword Genius

One refilling a pen instinctively? (8)

Ross

I believe the answer is:

sheepdog

I'm a little stuck... Click here to teach me more about this clue!

This clue might be a double definition.

'one refilling a' is the first definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'pen instinctively?' is the second definition.
The definition and answer can be both animals as well as being singular nouns.
Maybe you can see an association between them that I can't see?

Can you help me to learn more?

(Other definitions for sheepdog that I've seen before include "Collie, perhaps" , "Working farm animal" , "Collie, for example" , "Pastoral assistant?" , "Canine trained to control flock" .)

I've seen this clue in The Independent.
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