A word that describes itself is called autological word or autonym (e.g., "word" is a word, "English" is an English word). The grid spanner 39A has struck me with the above quality so I started from it and looked for other autonyms. 18A and 60A are not only symmetrical but also concern linguistics duo: semantics and syntax respectively. Hope you like clues for 25A, 43A, 12D and 28D!