Part of crustacean or old, old, group of whales (6)
I believe the answer is:
exopod
'part of crustacean' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.
'old old group of whales' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'group of whales' becomes 'pod' (term for a group of whales).
'ex'+'o'+'pod'='EXOPOD'
'or' is the link.
Can you help me to learn more?
(Another definition for exopod that I've seen is " one out on a limb".)