Simple defence group dared no improvement (9)
I believe the answer is:
unadorned
'simple' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for unadorned that I've seen before include "Without embellishment" , "Plain, undecorated" , "Not decorated, plain and simple" , "Simple and plain as around den" .)