Pure substance dissolved in addition to salt (8)
I believe the answer is:
absolute
'pure' is the definition.
(similar in meaning)
'substance dissolved in addition to salt' is the wordplay.
'substance dissolved' becomes 'solute' (substance which can be dissolved).
'in addition to' means one lot of letters go next to another.
'salt' becomes 'ab' (able seaman - 'salt' can mean a sailor).
'solute' after 'ab' is 'ABSOLUTE'.
(Other definitions for absolute that I've seen before include "Perfect, complete, pure" , "Complete or not limited" , "Total; not relative" , "Not qualified or diminished in any way" , "Rival" .)