Unaware of plain surrounding lake and island (9)
I believe the answer is:
oblivious
'unaware of' is the definition.
(I've seen this before)
'plain surrounding lake and island' is the wordplay.
'plain' becomes 'obvious' (similar in meaning).
'surrounding' means one lot of letters goes inside another.
'lake' becomes 'L' (geographical abbreviation).
'and' is a charade indicator (letters next to each other).
'island' becomes 'i' (geographical abbreviation).
'l'+'i'='li'
'obvious' going around 'li' is 'OBLIVIOUS'.
(Other definitions for oblivious that I've seen before include "totally in the dark?" , "Not noticing" , "like Lethe?" , "Totally unaware" , "Blind (to)" .)