Adamant that aim is to protect sibling (9)

I believe the answer is:

insistent

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'adamant' is the definition.
Both the answer and definition are adjectives. Perhaps you can see a link between them that I don't see?

'aim is to protect sibling' is the wordplay.
'aim' becomes 'intent' (synonyms).
'is to protect' indicates putting letters inside.
'sibling' becomes 'sis' (short for sister).
'intent' placed around 'sis' is 'INSISTENT'.

'that' is the link.

Can you help me to learn more?

(Other definitions for insistent that I've seen before include "Unwilling to take "no" for an answer" , "Not giving up" , "Refusing to be thwarted" , "Emphatic" , "Compelling attention" .)

I've seen this clue in the Sydney Morning Herald.

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