The most common and most likely answer to this crossword clue is the 4 letter word PARR
I've seen this crossword clue in The Guardian
P
A
R
R
More clues leading to the result PARR
- Queen Catherine somewhat stuck up, arrogant
- Average king's sixth wife? Something fishy here
- Salmon, royal survivor of the sixteenth century?
- . Standard recipe for salmon
- Standard river fish
- Fish pie and raspberry ripple starters
- Salmon caught by sparrowhawk
- Fishwife who survived?
- Standard piece of rock salmon
- Fish wife?
Buddy explains!

The straight part
We know that "Fish/wife (last of 6)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fish/wife (last of 6) = parr.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 PARR mean?
PARR noun- Young salmon, at a stage between fry and smolt when they feed chiefly on invertebrates but cannot tolerate saltwater.
- A young leveret.