Element illuminated with buzz holding current (7)
I believe the answer is:
Here is my best explanation:
'element' is the definition.
(lithium is a kind of element)
'illuminated with buzz holding current' is the subsidiary indication.
'illuminated' becomes 'lit' ('lit' can be a synonym of 'illuminated').
'with' says to put letters next to each other.
'buzz' becomes 'hum' ('hum' can be a synonym of 'buzz').
'holding' is an insertion indicator.
'current' becomes 'i' (symbol for electric current).
'hum' enclosing 'i' is 'hium'.
(Other definitions for lithium that I've seen before include "Soft silvery element", "Element used in batteries", "Lightest-known metal", "Light metal element", "Silver-white metal (symbol: Li)", "Lightest metallic element", "Light metallic element", "Chemical element (Li), used in batteries" and "Lightest alkali metal".)