Unaware, left island blocking patent (9)
I believe the answer is:
oblivious
'unaware' is the definition.
('oblivious' can be similar in meaning to 'unaware')
'left island blocking patent' is the wordplay.
'left' becomes 'l' (common abbreviation).
'island' becomes 'i'.
'blocking' means one lot of letters goes inside another.
'patent' becomes 'obvious' ('obvious' can be similar in meaning to 'patent').
'l'+'i'='li'
'li' put within 'obvious' is 'OBLIVIOUS'.
(Other definitions for oblivious that I've seen before include "Totally unaware" , "Apt to forget" , "like Lethe?" , "Not noticing" , "Ignorant" .)