The most common and most likely answer to this crossword clue is the 4 plus 7 letter word WOLF WHISTLE
I've seen this crossword clue in The Times
W
O
L
F
W
H
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S
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More clues leading to the result WOLF WHISTLE
Buddy explains!
The straight part
We know that "What may be blown from building site" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what may be blown from building site = wolf whistle.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 WOLF WHISTLE mean?
WOLF WHISTLE noun- A whistle, of two notes, to signify sexual interest in someone.