The most common and most likely answer to this crossword clue is the 3 letter word EMU
I've seen this crossword clue in The Times
E
M
U
More clues leading to the result EMU
- Grounded winger involved in three murders
- Bird Science Museum exhibits
- Bird frequenting quagmire, mudlark
- Bird in science unit
- Teams up every so often to produce large flyer? No!
- Rival drops slow Aussie
- Flightless bird originally eliciting letter from abroad
- Bird stuck in the mud
- Single currency agreement, one that can’t get off the ground
- Bird: I'm not sure able in the end to soar?
Buddy explains!

The straight part
We know that "Bird" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bird = emu.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 EMU mean?
EMU noun- A cassowary (genus Casuarius).
- A large flightless bird native to Australia, Dromaius novaehollandiae.
- Any of the various units to measure electricity and magnetism in the CGS (now replaced by SI) system of units; abbreviated emu or EMU.
- A person or thing that emulates.
- A piece of software or hardware that simulates another system.