The most common and most likely answer to this crossword clue is the 10 letter word UNTHANKFUL
I've seen this crossword clue in The Guardian
U
N
T
H
A
N
K
F
U
L
Buddy explains!
The straight part
We know that "showing no appreciation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case showing no appreciation = unthankful.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 UNTHANKFUL mean?
UNTHANKFUL adjective- Not thankful; ungrateful.