Opening without uniform for flier (4)

I believe the answer is:

moth

I'm a little stuck... Click here to teach me more about this clue!

'flier' is the definition.
(something which flies)

I can't explain the rest of the clue.

(Other definitions for moth that I've seen before include "night flyer" , "Night flying insect" , "Lepidopteran" , "Usually nocturnal insect" , "Night flier" .)

I've seen this clue in The Telegraph.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.