By the French, said to be the chief Greek island (6)
I believe the answer is:
lesbos
'island' is the definition.
(Greek island)
'by the french said to be the chief greek' is the wordplay.
'by' says to put letters next to each other.
'the french' becomes 'les' (plural form of 'the' in French).
'said' indicates a 'sounds like' (homophone) clue.
'the chief greek' becomes 'os' (I can't justify this - if you can you should believe this answer much more).
'be' sounds like 'b'.
'les' next to 'b' is 'lesb'.
'lesb'+'os'='LESBOS'
Can you help me to learn more?
(Other definitions for lesbos that I've seen before include "Greek island -- home of 3" , "Island, home of Sappho" , "Jot" , "Greek island, birthplace of Sappho" .)