A lieutenant lists first form of sodium (5,4)
I believe the answer is:
table salt
'sodium' is the definition.
(I know that sodium can be written as table salt)
'a lieutenant lists first' is the wordplay.
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'lists' becomes 'tables' (I am not sure about this - if you are sure you should believe this answer much more).
'first' says to put letters next to each other.
'a'+'lt'='alt'
'alt' put after 'tables' is 'TABLE SALT'.
'form of' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for table salt that I've seen before include "Seasoning for meal" , "Condiment" , "It adds zest to meals" , "item on board" , "white powder" .)