Defence group with connection to loosen (5)
I believe the answer is:
untie
'to loosen' is the definition.
(I've seen this before)
'defence group with connection' is the wordplay.
'defence group' becomes 'un' (I can't justify this - if you can you should believe this answer much more).
'with' means one lot of letters go next to another.
'connection' becomes 'tie' (tie is a kind of connection).
'un'+'tie'='UNTIE'
Can you help me to learn more?
(Other definitions for untie that I've seen before include "Set free" , "Separate" , "Release bindings or shoelaces" , "Undo, unfasten" , "resolve" .)