The most common and most likely answer to this crossword clue is the 4 letter word GILL
I've seen this crossword clue in Financial Times
G
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Buddy explains!

The straight part
We know that "Breathing apparatus" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case breathing apparatus = gill.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 GILL mean?
GILL noun- (animal anatomy) a breathing organ of fish and other aquatic animals
- (of a fish) a gill slit or gill cover
- One of the radial folds on the underside of the cap of a mushroom, on the surface of which the spore-producing organs are borne
- (animal anatomy) the fleshy flap that hangs below the beak of a fowl; a wattle
- The flesh under or about the chin; a wattle
- (spinning) one of the combs of closely ranged steel pins which divide the ribbons of flax fiber or wool into fewer parallel filaments
- To remove the gills from a fish as part of gutting and cleaning it
- To catch (a fish) in a gillnet
- To be or become entangled in a gillnet
- A drink measure for spirits and wine (size varies regionally but it is about one quarter of a pint)
- A measuring jug holding a quarter or half a pint
- Rivulet
- Ravine
- A two-wheeled frame for transporting timber
- A leech
- A female ferret
- A promiscuous woman; harlot, wanton
- A prostitute
- To act as a prostitute.