Clue: What's said to be terrible root crop or plant (5-4)
Answer: DYER’S WEED
Explanation: Definition: plant
A charade of DYER, sounding like ('what's said') DIRE ('terrible') plus SWEED, again sounding like (eliding the word break, and ignoring the s-z sound difference) SWEDE ('root crop'; it's called a rutabaga in this neck of the woods), for a plant, woad perhaps.