The most common and most likely answer to this crossword clue is the 4 letter word IBIS
I've seen this crossword clue in The Times
I
B
I
S
More clues leading to the result IBIS
- Bird, single, 21
- Bird, one going over British Isles
- Wading bird, one initially breeding in swamps
- Heading for insolvency again, one has a large bill
- Bird is catching bird? No way!
- Wader again seen on island
- Hotel group is accommodating AC/DC
- … and one principally inhabiting British Isles
- Wader twice spotted near island
- Wader in boots is struggling initially
Buddy explains!

The straight part
We know that "Wader" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case wader = ibis.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 IBIS mean?
IBIS noun- Any of various long-legged wading birds in the family Threskiornithidae, having long downcurved bills used to probe the mud for prey such as crustaceans.