The most common and most likely answer to this crossword clue is the 4 letter word FLUX
I've seen this crossword clue in The Guardian
F
L
U
X
Buddy explains!
The straight part
We know that "Ongoing mutation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ongoing mutation = flux.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 FLUX mean?
FLUX noun- The act of flowing; a continuous moving on or passing by, as of a flowing stream.
- A state of ongoing change.
- A chemical agent for cleaning metal prior to soldering or welding.
- The rate of transfer of energy (or another physical quantity) through a given surface, specifically electric flux, magnetic flux.
- A disease which causes diarrhea, especially dysentery.
- Diarrhea or other fluid discharge from the body.
- The state of being liquid through heat; fusion.
- To use flux on.
- To melt.
- To flow as a liquid.
- Flowing; unstable; inconstant; variable.