In the flower stalk a little chemical substance is present (6)
I believe the answer is:
alkali
'chemical substance is present' is the definition.
I can't judge whether this definition defines the answer.
'in the flower stalk a little' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ALKALI' is hidden in the letters of 'flower stalk a little'.
Can you help me to learn more?
(Other definitions for alkali that I've seen before include "Antacid" , "Non-acid" , "Substance such as lime or caustic soda" , "Substance that neutralises acid" , "Acid-neutralising substance" .)