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1d Frenzied , eating doughnut or tropical root . ?a?i?c . Must be maniac but can't see where the doughnut or tropical root come in .

I think it's MANIOC (a tropical root). MANIC with O (doughnut?) in it.

I think you are right -never heard of manioc. Incidentally the crossword was a slog with lots of clues referring to 6d and many others cross referencing others .
On the basis that a word is obscure if I've never heard of it, the other obscurities were
5d Blimey! Arab ib crumpled hat shows urge to fib (10)
29a Roman deity in a pile curiously , of seabed creatures (9)

I used solvers for them.

manioc is better known as cassava