The most common and most likely answer to this crossword clue is the 7 letter word TRIFFID
T
R
I
F
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Buddy explains!

The straight part
We know that "fictional plant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fictional plant = triffid.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does TRIFFID mean?
TRIFFID noun- A fictional plant, able to move around and kill people with a poisonous stinger
- Any of the anemones of the genus Aiptasia with poisonous tentacles, which are a pest in aquaria.
- Any large, fast-growing and hard-to-remove weed, such as the flowering shrub Chromolaena odorata.