Rainbow warship carrying very large, and thickening, agent (5,4)
I believe the answer is:
irish moss
'thickening agent' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'rainbow warship carrying very large' is the wordplay.
'rainbow' becomes 'iris' (I've seen this in another clue).
'warship' becomes 'hms' (I am not sure about this - if you are sure you should believe this answer much more).
'carrying' is an insertion indicator.
'very large' becomes 'os' (abbreviation for oversize).
'hms' going around 'os' is 'hmoss'.
'iris'+'hmoss'='IRISH MOSS'
'and' is the link.
Can you help me to learn more?
(Other definitions for irish moss that I've seen before include "Edible seaweed, carrageen" , "Edible red seaweed" .)