Group under attack puts off accepting downfall (9)
I believe the answer is:
defenders
'group under attack' is the definition.
'puts off accepting downfall' is the wordplay.
'puts off' becomes 'defers' ('defer' can be a synonym of 'put off').
'accepting' indicates putting letters inside.
'downfall' becomes 'end'.
'defers' enclosing 'end' is 'DEFENDERS'.
(Other definitions for defenders that I've seen before include "They support" , "Guardians, protectors" .)