The most common and most likely answer to this crossword clue is the 8 letter word BLANDISH
I've seen this crossword clue in The Times
B
L
A
N
D
I
S
H
Buddy explains!

The straight part
We know that "Flatter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flatter = blandish.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 BLANDISH mean?
BLANDISH verb- To persuade someone by using flattery; to cajole.
- To praise someone dishonestly; to flatter or butter up.