The most common and most likely answer to this crossword clue is the 7 plus 7 letter word TU-WHIT TU-WHOO
I've seen this crossword clue in Financial Times
T
U
-
W
H
I
T
T
U
-
W
H
O
O
Buddy explains!
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.