The most common and most likely answer to this crossword clue is the 8 letter word IMPOSTER
I've seen this crossword clue in Financial Times
I
M
P
O
S
T
E
R
More clues leading to the result IMPOSTER
- The writer’s online chatter, maybe, is a sham
- Fake claim by online commenter?
- This writer’s one contributing to blog, a fraud
- I’m Bill and I’m not what I seem
- Quack is briefly married to Bill
- An identity thief, Neo’s following the monarch
- Young offender misled store in fraud
- I am Bill and I make false claims
- Deceitful character‘s broken promise to cut back
- Fake the compiler’s advertisement
Buddy explains!

The straight part
We know that "An identity thief" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an identity thief = imposter.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 IMPOSTER mean?
IMPOSTER noun- Someone who attempts to deceive by using an assumed name or identity.
- A sprite or animation integrated into a three-dimensional scene, but not based on an actual 3D model.