The most common and most likely answer to this crossword clue is the 5 plus 2 letter word WATCH IT
I've seen this crossword clue in The Daily Telegraph
W
A
T
C
H
I
T
More clues leading to the result WATCH IT
Buddy explains!

The straight part
We know that "Be careful" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case be careful = watch it.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.