Poison diffused at this point somewhere up high (10)
I believe the answer is:
ionosphere
'somewhere up high' is the definition.
The answer and definition can be both locations as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'poison diffused at this point' is the wordplay.
I cannot quite see how this works, but
an anagram of 'poison' is 'ionosp' which is present in the answer.
'at' could be 'her' (I've seen this in other clues) and 'her' is located in the answer.
'point' could be 'e' (compass point) and 'e' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for ionosphere that I've seen before include "distant region" , "gaseous envelope?" , "Where to find Appleton and Heaviside layers" , "Layer reflecting radio waves" , "Elevated region" .)