Urgent aim to protect little sibling (9)
I believe the answer is:
insistent
'urgent' is the definition.
(synonyms)
'aim to protect little sibling' is the wordplay.
'aim' becomes 'intent' (synonyms).
'to protect' is an insertion indicator.
'little sibling' becomes 'sis' (short for sister).
'intent' enclosing 'sis' is 'INSISTENT'.
(Other definitions for insistent that I've seen before include "Determined" , "Not giving up" , "Declaring or demanding emphatically" , "Constant" , "Emphatic" .)