Bitter flavouring upset Argonauts (9)
I believe the answer is:
angostura
'bitter flavouring' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'upset argonauts' is the wordplay.
'upset' indicates anagramming the letters.
'argonauts' with letters rearranged gives 'ANGOSTURA'.
Can you help me to learn more?
(Other definitions for angostura that I've seen before include "Bitter stuff for Argonauts" , "Bitters for 13" , "Bitter bark of a South American tree, used in liqueurs" , "An aromatic bark, made bitter tonic" , "bitter substance" .)