The most common and most likely answer to this crossword clue is the 9 letter word TRA-LA-LA
I've seen this crossword clue in The Guardian
T
R
A
-
L
A
-
L
A
Buddy explains!

The straight part
We know that "Refr/ai/n" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case refr/ai/n = tra-la-la.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 TRA-LA-LA mean?
TRA-LA-LA interjection- Meaningless syllables used when humming a tune