Unconscious, having no lithium that's clear (7)
I believe the answer is:
obvious
'clear' is the definition.
(similar in meaning)
'unconscious having no lithium' is the wordplay.
'unconscious' becomes 'oblivious' (I've seen this before).
'having no' indicates named letters should be taken away.
'lithium' becomes 'Li' (chemical symbol).
'oblivious' with 'li' taken out is 'OBVIOUS'.
'that's' acts as a link.
(Other definitions for obvious that I've seen before include "Quite evident, manifest" , "Readily seen" , "Plainly seen" , "Apparent" , "Self-evident" .)