Not a new defence group (4)
I believe the answer is:
nato
'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.
'not'+'a'='nota'
'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" .)