Retreat to north to cover country devastatingly attacked (5)
I believe the answer is:
nuked
'devastatingly attacked' is the definition.
(to nuke is to attack with a nuclear bomb)
'retreat to north to cover country' is the wordplay.
'retreat' becomes 'den'.
'to north' says the letters should be written backwards (in a down clue, letters go up or 'northward').
'to cover' is an insertion indicator.
'country' becomes 'uk' (UK is an example).
'den' written backwards gives 'ned'.
'ned' enclosing 'uk' is 'NUKED'.
(Other definitions for nuked that I've seen before include "zapped into oblivion?" , "Used bombs" .)