Fantastic plant's amazing, finally five times bigger (7)
I believe the answer is:
triffid
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for triffid that I've seen before include "John Wyndham's lethal walking plant" , "John Wyndham's alien plant" , "novel poisoner" , "Killing growth" , "Predatory sci-fi plant" .)