The most common and most likely answer to this crossword clue is the 3 plus 5 plus 3 plus 3 letter word MAD ABOUT THE BOY
I've seen this crossword clue in The Daily Telegraph
M
A
D
A
B
O
U
T
T
H
E
B
O
Y
More clues leading to the result MAD ABOUT THE BOY
Buddy explains!
The straight part
We know that "popular song" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case popular song = mad about the boy.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.