Have your confusions sorted right here.
Update: My confusions:
15 across: Hindu destroyer fairly vital to twin (9)
I think I worked out the answer, just not the explanation for the last two letters of what I thought was the answer.
21 down: Porter of Australian letters suggested call to hunters (6)
5 down: Guide outspoken German author and his literary output? (9)
12 across: Mulched seaport grass (7)
I don’t know my grasses.
6 down: They mark the old gurus heading last (3, 5)