Terrific tuna cooked by one of the family (5-4)
I believe the answer is:
great-aunt
'one of the family' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'terrific tuna cooked by' is the wordplay.
'terrific' becomes 'great' (I've seen this before).
'cooked' is an anagram indicator (letters cooked into a new form).
'by' means one lot of letters go next to another.
'tuna' with letters rearranged gives 'aunt'.
'great'+'aunt'='GREAT-AUNT'
Can you help me to learn more?
(Another definition for great-aunt that I've seen is " Tag nature (anag) - relative".)