The most common and most likely answer to this crossword clue is the 5 letter word KAPOK
I've seen this crossword clue in Financial Times
K
A
P
O
K
Buddy explains!

The straight part
We know that "filler" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case filler = kapok.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 KAPOK mean?
KAPOK noun- A silky fibre obtained from the silk-cotton tree used for insulation and stuffing for pillows, mattresses, etc.