Finally, stir in fizzy tonic and lemon flavouring (6)
I believe the answer is:
citron
'lemon flavouring' is the definition.
The answer and definition can be both things that one consumes for nourishment as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'finally stir in fizzy tonic' is the wordplay.
'finally' indicates one should take the final letters.
'in' is an insertion indicator.
'fizzy' indicates anagramming the letters (I've seen this in other clues).
The final letter of 'stir' is 'r'.
'tonic' is an anagram of 'citon'.
'r' going into 'citon' is 'CITRON'.
'and' acts as a link.
Can you help me to learn more?
(Other definitions for citron that I've seen before include "Large lemon-like fruit" , "Tree producing fruit" , "It's like a lemon" .)