Salt measure added to pastry (8)
I believe the answer is:
tartrate
'salt' is the definition.
(I know that tartrate is a type of salt)
'measure added to pastry' is the wordplay.
'measure' becomes 'rate' (rating is a kind of measuring).
'added to' says to put letters next to each other.
'pastry' becomes 'tart' (type of pastry).
'rate' after 'tart' is 'TARTRATE'.
(Other definitions for tartrate that I've seen before include "compound?" , "salt" .)