Salt — a partner after excessive amount (9)
I believe the answer is:
glutamate
'salt' is the definition.
(glutamate is a kind of salt)
'a partner after excessive amount' is the wordplay.
'partner' becomes 'mate' ('mate' can be a synonym of 'partner'**).
'after' means one lot of letters go next to another.
'excessive amount' becomes 'glut' (I've seen this before).
'a'+'mate'='amate'
'amate' after 'glut' is 'GLUTAMATE'.
(Other definitions for glutamate that I've seen before include "Flavour enhancer" , "Salt" .)