The most common and most likely answer to this crossword clue is the 6 letter word PARROT
I've seen this crossword clue in The Times
P
A
R
R
O
T
More clues leading to the result PARROT
Buddy explains!

The straight part
We know that "Imitator" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case imitator = parrot.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 PARROT mean?
PARROT noun- A kind of bird, many species of which are colourful and able to mimic human speech, of the order Psittaciformes or (narrowly) of the family Psittacidae.
- A parroter; a person who repeats the words or ideas of others.
- A puffin.
- Channel coal.
- A transponder.
- To repeat (exactly what has just been said) without necessarily showing understanding, in the manner of a parrot.