The most common and most likely answer to this crossword clue is the 6 letter word SALTER
I've seen this crossword clue in The Daily Telegraph
S
A
L
T
E
R
Buddy explains!
The straight part
We know that "one for preservation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one for preservation = salter.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 SALTER mean?
SALTER noun- One who makes, sells, or applies salt.
- A trout leaving salt water to ascend a stream.