The most common and most likely answer to this crossword clue is the 7 letter word STEALER
I've seen this crossword clue in The Daily Telegraph
S
T
E
A
L
E
R
More clues leading to the result STEALER
Buddy explains!
The straight part
We know that "Thief" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case thief = stealer.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 STEALER mean?
STEALER noun- (chiefly in combination) One who steals; a thief.
- The endmost plank of a strake which stops short of the stem or stern.