The most common and most likely answer to this crossword clue is the 3 plus 5 plus 5 plus 5 plus 5 plus 5 plus 2 letter word PUT ONE’S MONEY WHERE ONE’S MOUTH IS
I've seen this crossword clue in The Guardian
P
U
T
O
N
E
â
€
™
S
M
O
N
E
Y
W
H
E
R
E
O
N
E
â
€
™
S
M
O
U
T
H
I
S
Buddy explains!
The straight part
We know that "To demonstrate one’s sincerity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to demonstrate one’s sincerity = put one’s money where one’s mouth is.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.