The most common and most likely answer to this crossword clue is the 8 letter word INFUSING
I
N
F
U
S
I
N
G
More clues leading to the result INFUSING
Buddy explains!

The straight part
We know that "filling" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case filling = infusing.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 INFUSING mean?
INFUSING verb- To cause to become an element of something; to insert or fill.
- To steep in a liquid, so as to extract the soluble constituents (usually medicinal or herbal).
- To inspire; to inspirit or animate; to fill (with).
- To instill as a quality.
- To undergo infusion.
- To make an infusion with (an ingredient); to tincture; to saturate.
- To pour in, as a liquid; to pour (into or upon); to shed.