Joke about weight taken the wrong way — toxic stuff! (5)
I believe the answer is:
ozone
'toxic stuff' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'joke about weight taken the wrong way' is the wordplay.
'joke' becomes 'one' (as in 'that's a good one!').
'about' means one lot of letters goes inside another.
'weight' becomes 'oz'.
'taken the wrong way' says the letters should be written in reverse.
'oz' backwards is 'zo'.
'one' enclosing 'zo' is 'OZONE'.
Can you help me to learn more?
(Other definitions for ozone that I've seen before include "Condensed oxygen; sea air" , "Bracing seaside air" , "something refreshing" , "Clean, bracing air" , "Toxic form of oxygen" .)