Unfortunate, losing old dog to a shark (9)

I believe the answer is:

porbeagle

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'shark' is the definition.
(I know that porbeagle is a type of mackerel shark)

'unfortunate losing old dog' is the wordplay.
'unfortunate losing old' becomes 'por' (I am not sure about this - if you are sure you should believe this answer much more).
'dog' becomes 'beagle' (beagle is a kind of dog).
'por'+'beagle'='PORBEAGLE'

'to a' is the link.

Can you help me to learn more?

(Other definitions for porbeagle that I've seen before include "Swimmer" , "Shark" .)

I've seen this clue in The Daily Express.

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