8 down: 21, 13-down (natural Tasmanian) sowed and planted during thinning process (7)
The answer here is cundall, as in Peter Cundall, the Tasmanian horticulturalist. I just can’t figure out why.
3 down: Glow briefly engulfs key English director (5)
Mike Leigh is an English director, so leigh = English director makes sense, but I can’t quite figure the rest out even though glow = light and glow briefly = ligh sounds about right.
26 across: Spicy as 13-down’s (Tasmanian’s) home? (8)
The answer is devilled, and I’m kind of getting a reference to a Tasmanian devil and a town or ville, but it’s not completely watertight an explanation.