Dog you could call "Rocky"? (5)

I believe the answer is:

cairn

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'dog' is the definition.
(I know that cairn is a type of dog)

I cannot understand how the remainder of the clue works.

(Other definitions for cairn that I've seen before include "Piled-stone marker" , "dog" , "Stones piled up as a marker" , "Variety of 10" , "Pile of stones - type of terrier" .)

I've seen this clue in the Evening Standard.

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