Golden retriever in this mythical ship? (4)

I believe the answer is:

argo

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'mythical ship?' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

(Other definitions for argo that I've seen before include "Name of ship sailed by Jason in search of the Golden Fleece" , "Famous ship" , "Fabulous ship" , "Ship Jason sailed to find the Golden Fleece" , "Stars once recognised in the south" .)

I've seen this clue in The Sun.

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