The most common and most likely answer to this crossword clue is the 7 letter word PHONIES
I've seen this crossword clue in The Guardian
P
H
O
N
I
E
S
Buddy explains!

The straight part
We know that "Impostors" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case impostors = phonies.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 PHONIES mean?
PHONIES noun- A person who assumes an identity or quality other than their own.
- A person who professes beliefs or opinions that they do not hold.
- Anything fraudulent or fake.