Seaweed found by idiot aboard small boat in Greek legend (8)
I believe the answer is:
sargasso
'seaweed' is the definition.
(I've seen this before)
'idiot aboard small boat in greek legend' is the wordplay.
'idiot' becomes 'ass' (both can mean a stupid person).
'aboard' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'boat in greek legend' becomes 'argo' (I can't explain this - if you can you should believe this answer much more).
's'+'argo'='sargo'
'ass' placed into 'sargo' is 'SARGASSO'.
'found by' acts as a link.
Can you help me to learn more?
(Other definitions for sargasso that I've seen before include "Gas soars around the North Atlantic sea" , "Sea with much floating weed" , "Sea in the North Atlantic with no land borders" , "Atlantic sea surrounded by ocean currents" , "Mass of weed" .)