The most common and most likely answer to this crossword clue is the 5 plus 2 plus 7 letter word BIRDS OF PASSAGE
I've seen this crossword clue in The Sunday Telegraph
B
I
R
D
S
O
F
P
A
S
S
A
G
E
Buddy explains!
The straight part
We know that "Transient visitors" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case transient visitors = birds of passage.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.