The trouble's about a chemical (4)
I believe the answer is:
soda
'a chemical' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'the trouble's about' is the wordplay.
'the trouble's' becomes 'ados' (both can mean bother or commotion).
'about' says the letters should be written in reverse.
'ados' backwards is 'SODA'.
Can you help me to learn more?
(Other definitions for soda that I've seen before include ""Splashed" drinks mixer" , "kind of water" , "Alkaline substance used in cleaning, drinks etc" , "Mixer drink" , "fizz" .)