Without potassium, sodium makes up salt (6)
I believe the answer is:
seaman
'salt' is the definition.
(synonyms)
'without potassium sodium makes up' is the wordplay.
'without' is a deletion indicator.
'potassium' becomes 'K' (K is the chemical symbol for potassium).
'sodium' becomes 'NA' (chemical symbol).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'na'+'makes'='namakes'
'namakes' with 'k' taken away is 'namaes'.
'namaes' in reverse letter order is 'SEAMAN'.
(Other definitions for seaman that I've seen before include "crew member" , "A manse (anag.)" , "Mariner" , "Sailor" .)