A bitter draught for assembled Argonauts (9)
I believe the answer is:
angostura
'a bitter draught' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'assembled argonauts' is the wordplay.
'assembled' is an anagram indicator.
'argonauts' is an anagram of 'ANGOSTURA'.
'for' is the link.
Can you help me to learn more?
(Other definitions for angostura that I've seen before include "Bitter bark used as a flavouring" , "Bitters for 13" , "bitter substance" , "special flavour" , "bitter source" .)