The most common and most likely answer to this crossword clue is the 10 letter word INFALLIBLE
I've seen this crossword clue in Independent
I
N
F
A
L
L
I
B
L
E
Buddy explains!

The straight part
We know that "perfect" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case perfect = infallible.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 INFALLIBLE mean?
INFALLIBLE adjective- Without fault or weakness; incapable of error or fallacy.
- Certain to produce the intended effect, sure.