Shark is unfortunate, missing old dog (9)
I believe the answer is:
porbeagle
'shark' is the definition.
(I know that porbeagle is a type of mackerel shark)
'unfortunate missing old dog' is the wordplay.
'unfortunate missing 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'
'is' is the link.
Can you help me to learn more?
(Other definitions for porbeagle that I've seen before include "Shark" , "Swimmer" .)