The most common and most likely answer to this crossword clue is the 4 letter word ABLE
I've seen this crossword clue in The Times
A
B
L
E
More clues leading to the result ABLE
- Efficient medical treatment out of bounds
- Clever message concealing its opening
- Expert turning up in hotel bar
- No time for food, getting fit
- Renovated Milanese abode with bad rating
- Expert bending shortened steel bars
- Competent brewer initially brought in beer
- Fit porter, maybe, carrying bishop
- Clever Biblical character bringing up learner
- Expert detaching first bit of wire
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 = able.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 ABLE mean?
ABLE adjective- Easy to use.
- Suitable; competent.
- Liable to.
- Having the necessary powers or the needed resources to accomplish a task.
- Free from constraints preventing completion of task; permitted to; not prevented from.
- Having the physical strength; robust; healthy.
- Rich; well-to-do.
- Gifted with skill, intelligence, knowledge, or competence.
- Legally qualified or competent.
- Capable of performing all the requisite duties; as an able seaman.
- To make ready.
- To make capable; to enable.
- To dress.
- To give power to; to reinforce; to confirm.
- To vouch for; to guarantee.
- The letter "A" in Navy Phonetic Alphabet.