The most common and most likely answer to this crossword clue is the 5 letter word SOUND
I've seen this crossword clue in The Guardian
S
O
U
N
D
Buddy explains!

The straight part
We know that "Fit/Fit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fit/fit = sound.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 SOUND mean?
SOUND adjective- Healthy.
- Complete, solid, or secure.
- Having the property of soundness.
- Good; acceptable; decent.
- (of sleep) Quiet and deep. Sound asleep means sleeping peacefully, often deeply.
- Heavy; laid on with force.
- Founded in law; legal; valid; not defective.
- Soundly.
- Yes; used to show agreement or understanding, generally without much enthusiasm.
- A sensation perceived by the ear caused by the vibration of air or some other medium.
- A vibration capable of causing such sensations.
- A distinctive style and sonority of a particular musician, orchestra etc
- Noise without meaning; empty noise.
- Earshot, distance within which a certain noise may be heard.
- To produce a sound.
- To convey an impression by one's sound.
- To be conveyed in sound; to be spread or published; to convey intelligence by sound.
- To resound.
- (often with in) To arise or to be recognizable as arising in or from a particular area of law.
- To cause to produce a sound.
- (of a vowel or consonant) To pronounce.
- A long narrow inlet, or a strait between the mainland and an island; also, a strait connecting two seas, or connecting a sea or lake with the ocean.
- The air bladder of a fish.
- A cuttlefish.
- An instrument for probing or dilating; a sonde.
- A long, thin probe for sounding body cavities or canals such as the urethra.
- Dive downwards, used of a whale.
- To ascertain, or try to ascertain, the thoughts, motives, and purposes of (a person); to examine; to try; to test; to probe.
- Test; ascertain the depth of water with a sounding line or other device.
- To examine with the instrument called a sound or sonde, or by auscultation or percussion.