The most common and most likely answer to this crossword clue is the 4 letter word AGUE
I've seen this crossword clue in The Times
A
G
U
E
More clues leading to the result AGUE
- Row away from river to get fit
- Fit date seen outside Uni
- First couple in capital have escaped fever
- A good universal cure ultimately for malarial fever
- Physical problem you finally found in nasty cage with pain as before
- Head off unspecified fever
- Lacking definition, not very fit
- Fit to dispute losing right
- Sickness from epidemic 18 got rid of
- Row, shunning river, to get fit
Buddy explains!

The straight part
We know that "Fit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fit = ague.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 AGUE mean?
AGUE noun- An acute fever.
- An intermittent fever, attended by alternate cold and hot fits.
- The cold fit or rigor of the intermittent fever
- A chill, or state of shaking, as with cold.
- Malaria.
- To strike with an ague, or with a cold fit.