The most common and most likely answer to this crossword clue is the 8 letter word THOROUGH
I've seen this crossword clue in The Daily Telegraph
T
H
O
R
O
U
G
H
More clues leading to the result THOROUGH
- Meticulous from beginning to end, assuming nothing
- Comprehensive, however, uncivilised
- All-embracing love, during hugging
- Utter nothing in between
- House with trench on the perimeter, detailed
- . Complete as a result of embracing love
- Love to impale non-stop, being conscientious
- Scrupulous but informal with thug
- Complete, from beginning to end, nothing concealed
- Principally, outrageous cuts finished comprehensive
Buddy explains!

The straight part
We know that "Careful" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case careful = thorough.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 THOROUGH mean?
THOROUGH noun- A furrow between two ridges, to drain off the surface water.
- Painstaking and careful not to miss or omit any detail.
- Utter; complete; absolute.
- Through.