Unaware manifest includes lithium (9)
I believe the answer is:
oblivious
'unaware' is the definition.
(similar in meaning)
'manifest includes lithium' is the wordplay.
'manifest' becomes 'obvious' (similar in meaning).
'includes' is an insertion indicator.
'lithium' becomes 'Li' (chemical symbol).
'obvious' enclosing 'li' is 'OBLIVIOUS'.
(Other definitions for oblivious that I've seen before include "Unconscious" , "Totally unaware" , "like Lethe?" , "Heedless" , "Apt to forget" .)