Knockout stuff from loch or ground (10)
I believe the answer is:
chloroform
'knockout stuff' is the definition.
'from loch or ground' is the wordplay.
'ground' is an anagram indicator ('grind' the letters into a new form).
'from'+'loch'+'or'='fromlochor'
'fromlochor' with letters rearranged gives 'CHLOROFORM'.
(Other definitions for chloroform that I've seen before include "Old anaesthetic" , "Solvent" , "Former anaesthetic" , "Sweet-smelling anaesthetic" .)