16a. Posh type with 23 round sweets (7) T??????
17d. Fathead with troubles doesn't work (5)
18d. Tail of some terrestrial animal (5)
21a. A boxer graduates a hero (7) [2 words] A??????
23a. Some of these etch lead (3) S?E (could be SEE as it's inside the clue - but why?)
24a. Plant close to herbaceous border (5) [possibly SEDGE]