Food seasoning at intervals usually going into flattish cake (5,4)
'food seasoning' is the definition.
'at intervals usually going into flattish cake' is the wordplay.
'at intervals' means one should take alternating letters (letters are taken at regular intervals).
'going into' is an insertion indicator.
'flattish cake' becomes 'tablet' (a small block or cake of something).
The alternating letters of 'usually' are 'sal'.
'sal' going into 'tablet' is 'TABLE SALT'.
(Other definitions for table salt that I've seen before include "It adds zest to meals" , "item on board" , "white powder" , "contents of cellar" , "Condiment" .)