The most common and most likely answer to this crossword clue is the 5 letter word SMOLT
I've seen this crossword clue in The Guardian
S
M
O
L
T
More clues leading to the result SMOLT
Buddy explains!

The straight part
We know that "Salmon" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case salmon = smolt.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 SMOLT mean?
SMOLT adjective- Bright; serene.
- (of weather) Calm; fine; fair.
- Smooth and shining.
- A young salmon two or three years old, when it has acquired its silvery color.