The most common and most likely answer to this crossword clue is the 5 letter word APERY
I've seen this crossword clue in Financial Times
A
P
E
R
Y
Buddy explains!

The straight part
We know that "Imitation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case imitation = apery.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 APERY mean?
APERY noun- A place where apes are kept.
- The practice of aping; an apish action.