Blow from behind is astern (8)
I believe the answer is:
disaster
'blow' is the definition.
(thesaurus)
'from behind is astern' is the wordplay.
'from' indicates the answer is hidden within the clue.
'DISASTER' is hidden in the letters of 'behind is astern'.
(Other definitions for disaster that I've seen before include "Calamity, sudden great misfortune" , "Terrible calamity" , "Tragedy of asterids" , "not a success!" , "Heavy blow" .)