Argonauts set off in some sort of bark? (9)
I believe the answer is:
angostura
'some sort of bark?' is the definition.
'argonauts set off' is the wordplay.
'set off' indicates an anagram.
'argonauts' is an anagram of 'ANGOSTURA'.
'in' acts as a link.
(Other definitions for angostura that I've seen before include "Bitter flavouring for Argonauts" , "Bitter bark of a South American tree, used in liqueurs" , "An aromatic bark, made bitter tonic" , "bitter source" , "Bitter stuff for Argonauts" .)