The most common and most likely answer to this crossword clue is the 5 plus 7 letter word LAMBS LETTUCE
I've seen this crossword clue in The Times
L
A
M
B
S
L
E
T
T
U
C
E
More clues leading to the result LAMBS LETTUCE
Buddy explains!

The straight part
We know that "small plant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case small plant = lambs lettuce.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.