The most common and most likely answer to this crossword clue is the 5 letter word ROBIN
I've seen this crossword clue in The Times
R
O
B
I
N
More clues leading to the result ROBIN
- Bird with rook and duck on waste container
- Man shortly dressing formally
- Closely examining wingless bird
- Day was one men overturned skip
- Christmas bird takes a lot of covering
- Superhero sidekick runs over by trash can
- One tweeting point men put another way
- Flier that’s yellow turned up on container
- Chirpy sort almost putting on ceremonial dress
- Bird runs over waste receptacle
Buddy explains!

The straight part
We know that "Day was one" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case day was one = robin.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 ROBIN mean?
ROBIN noun- Any of various passerine birds (about 100 species) of the families Muscicapidae, Turdidae and Petroicidae (formerly Eopsaltriidae), typically with a red breast.
- A trimming in front of a dress.