I'd spelt out fully who I am (8)
I believe the answer is:
identity
'i am' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'i'd spelt out fully' is the wordplay.
'spelt out fully' becomes 'entity' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'id'+'entity'='IDENTITY'
'who' is the link.
Can you help me to learn more?
(Other definitions for identity that I've seen before include "it may precede crisis" , "papers" , "sort of card" , "Close affinity" , "Uniqueness" .)