The most common and most likely answer to this crossword clue is the 4 letter word WREN
I've seen this crossword clue in Financial Times
W
R
E
N
More clues leading to the result WREN
- Architect’s engineers blocking adversaries at bridge
- Serving woman's resistance during latest uprising
- Petite flyer no man in Royal Navy
- St Paul's man having traumatic experience with church lapsing
- Designer of buildings with reduced monthly charge?
- Old Brit associated with church — or Yank that no longer is
- . King gripped by novel about woman in service
- Architect’s painful separation having lost children
- Women’s backsides in border unnerve and frighten bird
- Novel about crossing rook with another bird
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 = wren.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 WREN mean?
WREN noun- Any member of a mainly New World passerine bird family Troglodytidae; true wren.
- Small bird of similar appearance to a true wren.