Defence group returned clues about centre (7)
I believe the answer is:
nucleus
'centre' is the definition.
(I know that nucleus is a type of centre)
'defence group returned clues about' is the wordplay.
'defence group' becomes 'un' (I can't explain this - if you can you should believe this answer much more).
'returned' says the letters should be written backwards.
'about' is an anagram indicator.
'un' back-to-front is 'nu'.
'clues' is an anagram of 'cleus'.
'nu'+'cleus'='NUCLEUS'
Can you help me to learn more?
(Other definitions for nucleus that I've seen before include "Tiny central part of an atom" , "Centre of growth" , "Central part of a cell" , "Central mass - kernel" , "central component" .)