BB re 18, it’s a composite (aka compound) anagram. Not a common device in blocked grids.
In a subtractive anagram the clue gives you the anagram fodder and what needs to be subtracted from it in order to generate the answer.
In a composite anagram the answer (more often than not indicated by “this” in the clue) plus other fodder from the clue can be used to form an anagram of another word from the clue (as here).