The most common and most likely answer to this crossword clue is the 5 letter word SMELT
I've seen this crossword clue in The Times
S
M
E
L
T
Buddy explains!

The straight part
We know that "Fish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fish = smelt.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 SMELT mean?
SMELT noun- Any small anadromous fish of the family Osmeridae, found in the Atlantic and Pacific Oceans and in lakes in North America and northern part of Europe.
- A fool; a simpleton.
- To sense a smell or smells.
- Followed by like or of if descriptive: to have a particular smell, whether good or bad.
- (without a modifier) To smell bad; to stink.
- To have a particular tincture or smack of any quality; to savour.
- To exercise sagacity.
- To detect or perceive; often with out.
- To give heed to.
- Production of metal, especially iron, from ore in a process that involves melting and chemical reduction of metal compounds into purified metal.
- Any of the various liquids or semi-molten solids produced and used during the course of such production.
- To fuse or melt two things into one, especially in order to extract metal from ore; to meld