The most common and most likely answer to this crossword clue is the 12 letter word HARTS-TONGUE
H
A
R
T
S
-
T
O
N
G
U
E
Buddy explains!
The straight part
We know that "tropical fern" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tropical fern = harts-tongue.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.