The most common and most likely answer to this crossword clue is the 6 letter word FINNED
I've seen this crossword clue in The Daily Telegraph
F
I
N
N
E
D
Buddy explains!
The straight part
We know that "like a fish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case like a fish = finned.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 FINNED mean?
FINNED verb- To cut the fins from a fish, shark, etc.
- (Of a fish) to swim with the dorsal fin above the surface of the water.
- To swim in the manner of a fish.
- To provide (a motor vehicle etc) with fins.
- Having fins.