The most common and most likely answer to this crossword clue is the 9 letter word HEARTFELT
I've seen this crossword clue in The Daily Telegraph
H
E
A
R
T
F
E
L
T
More clues leading to the result HEARTFELT
- Genuine theft, real criminal
- Deep earth left turned over
- Try and quit going around with delinquent, finally sincere
- Fervent father thrashed English lieutenant
- Courage perceived to be genuine
- Genuine chap maybe engraving on fibrous material
- Try temperature, port should be knocked back warm
- Outrageously flatter the man at the top — being sincere?
- Male with cunning thought sincere
- That man Garfunkel had an opinion that was meaningful
Buddy explains!

The straight part
We know that "being sincere?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case being sincere? = heartfelt.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 HEARTFELT mean?
HEARTFELT adjective- Felt or believed deeply and sincerely.