Stupid fellow is blocking most of Hawaiian island (6)
I believe the answer is:
oafish
'stupid' is the definition.
(I've seen this in another clue)
'fellow is blocking most of hawaiian island' is the wordplay.
'fellow' becomes 'F' (abbreviation - of a society etc.).
'blocking' indicates putting letters inside.
'most of' means to remove the last letter.
'hawaiian island' becomes 'oahu' (I've seen this before).
'f'+'is'='fis'
'oahu' with its last letter removed is 'oah'.
'fis' inserted within 'oah' is 'OAFISH'.
(Other definitions for oafish that I've seen before include "lacking elegance?" , "Boorish, clumsy" , "having poor manners" , "Stupid, boorish" , "Loutish" .)