Ring has additional twist - a chemical (9)
I believe the answer is:
pheromone
'chemical' is the definition.
Both the definition and answer are singular nouns.
Maybe there's an association between them I don't understand?
'ring has additional twist a' is the wordplay.
'ring has' becomes 'ph' (I can't explain this - if you can you should give a lot more credence to this answer).
'additional' becomes 'more' (synonyms).
'twist' says the letters should be written backwards (I've seen 'twisting' mean this (letters twist back)).
'a' becomes 'one'.
'more' in reverse letter order is 'erom'.
'ph'+'erom'+'one'='PHEROMONE'
Can you help me to learn more?
(Other definitions for pheromone that I've seen before include "Substance secreted that affects others of same species" , "Chemical affecting behaviour" , "Chemical attractor" , "Chemical signal between organisms" , "What attracts" .)