Not a new defence group (4)
I believe the answer is:
'defence group' is the definition.
The answer and definition can be both groups as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'not a new' is the wordplay.
'new' is an anagram indicator.
'nota' with letters rearranged gives 'NATO'.
Can you help me to learn more?
(Other definitions for nato that I've seen before include "Western mil. alliance" , "Military alliance, formed 1949" , "Military/political alliance" , "N. Atlantic alliance" , "Military organization" .)