The most common and most likely answer to this crossword clue is the 13 letter word IRREPLACEABLE
I've seen this crossword clue in The Daily Telegraph
I
R
R
E
P
L
A
C
E
A
B
L
E
More clues leading to the result IRREPLACEABLE
Buddy explains!

The straight part
We know that "Precious" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case precious = irreplaceable.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 IRREPLACEABLE mean?
IRREPLACEABLE adjective- That cannot be replaced, especially because it is unique.