Under attack, extremely tough Red Guards scoffed (10)
I believe the answer is:
threatened
'under attack' is the definition.
'extremely tough red guards scoffed' is the wordplay.
'extremely' means to remove the middle letters.
'guards' indicates putting letters inside.
'scoffed' becomes 'eaten' (I've seen this before).
'tough' with its centre taken out is 'th'.
'red' placed around 'eaten' is 'reatened'.
'th'+'reatened'='THREATENED'
(Other definitions for threatened that I've seen before include "Menaced" , "potentially facing extinction" , "Facing danger" , "Put at risk, endangered" , "was imminent" .)