The most common and most likely answer to this crossword clue is the 2 plus 4 plus 5 letter word IN GOOD FAITH
I've seen this crossword clue in The Daily Telegraph
I
N
G
O
O
D
F
A
I
T
H
Buddy explains!
The straight part
We know that "with perfect sincerity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case with perfect sincerity = in good faith.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.