The most common and most likely answer to this crossword clue is the 8 letter word CRANEFLY
I've seen this crossword clue in The Times
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More clues leading to the result CRANEFLY
Buddy explains!
The straight part
We know that "Leggy bird, alert, finding similar prey?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case leggy bird, alert, finding similar prey? = cranefly.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 CRANEFLY mean?
CRANEFLY noun- Any of various large flies of the suborder Tipulomorpha that have slender bodies and very long legs and that resemble giant mosquitoes.