The most common and most likely answer to this crossword clue is the 4 plus 5 letter word COLD FRAME
I've seen this crossword clue in The Guardian
C
O
L
D
F
R
A
M
E
More clues leading to the result COLD FRAME
Buddy explains!
The straight part
We know that "something to keep plants warm /something to keep plants warm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case something to keep plants warm /something to keep plants warm = cold frame.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.