Not seeing lake, I will cut through plain (9)
I believe the answer is:
oblivious
'not seeing' is the definition.
'lake i will cut through plain' is the wordplay.
'lake' becomes 'L' (geographical abbreviation).
'will cut through' means one lot of letters goes inside another (inserted letters cut into the word).
'plain' becomes 'obvious' (similar in meaning).
'l'+'i'='li'
'li' put into 'obvious' is 'OBLIVIOUS'.
(Other definitions for oblivious that I've seen before include "Blind (to)" , "Heedless" , "In ignorance" , "like Lethe?" , "Apt to forget" .)