Lithium's salted away in sea of projectiles (9)
I believe the answer is:
ballistic
'sea of projectiles' is the definition.
Although both the answer and definition are adjectives, I don't see how they can define each other.
'lithium's salted away in' is the wordplay.
'lithium's' becomes 'Li's' (chemical symbol).
'salted away' becomes 'baltic' (I can't explain this - if you can you should believe this answer much more).
'in' indicates putting letters inside.
'lis' inserted into 'baltic' is 'BALLISTIC'.
Can you help me to learn more?
(Other definitions for ballistic that I've seen before include "Like missiles" , "Winsome deer" , "Relating to projectiles and their flight" , "To do with the flight of projectiles - basic lilt (anag)" , "Moved by gravity" .)