Astonishing damage encompassed by nasty break and cut leg (10)
I believe the answer is:
remarkable
'astonishing' is the definition.
(thesaurus)
'damage encompassed by nasty break and cut leg' is the wordplay.
'damage' becomes 'mar' (to mar is to spoil or damage).
'encompassed by' means one lot of letters goes inside another.
'nasty' indicates anagramming the letters (a nasty or wrong spelling).
'and' says to put letters next to each other.
'cut' means to remove the last letter.
'break' anagrammed gives 'rekab'.
'mar' placed into 'rekab' is 'remarkab'.
'leg' with its final letter taken off is 'le'.
'remarkab'+'le'='REMARKABLE'
(Other definitions for remarkable that I've seen before include "Noteworthy" , "surprising?" , "A wrongly-graded exam paper" , "Striking" , "unusual" .)