The most common and most likely answer to this crossword clue is the 5 letter word MOLLY
I've seen this crossword clue in The Guardian
M
O
L
L
Y
Buddy explains!
The straight part
We know that "Fish/in weed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fish/in weed = molly.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 MOLLY mean?
MOLLY noun- A woman or girl, especially of low status.
- An effeminate male, a male homosexual.
- Pure MDMA powder.
- A mollemoke.
- A female cat, a she-cat (usually spayed)
- A bird, the wagtail.
- A molly bolt.
- To engage in (male) homosexual activity with.
- A fish of the genus Poecilia, except for those known as guppies.