The most common and most likely answer to this crossword clue is the 14 letter word UNDERESTIMATED
I've seen this crossword clue in The Times
U
N
D
E
R
E
S
T
I
M
A
T
E
D
Buddy explains!
The straight part
We know that "not properly appreciated" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not properly appreciated = underestimated.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 UNDERESTIMATED mean?
UNDERESTIMATED verb- To perceive (someone or something) as having a lower value, quantity, worth, etc., than what he/she/it actually has.