The most common and most likely answer to this crossword clue is the 8 letter word PERMEATE
I've seen this crossword clue in The Daily Telegraph
P
E
R
M
E
A
T
E
More clues leading to the result PERMEATE
- Go through food consumed by French dad
- Go through and wave at visiting twins in glee
- See me in repeat broadcast that’s diffuse
- Infiltrate team prepared to stop father in France
- Go through Russian city with European friend short of money
- For Moley a note to pass through
- Saturate with pressure before eating food
- Pass through a substance then echo
- Riddle for each partner embracing sweetheart
- Poorly peer eating beef perhaps to fill up
Buddy explains!

The straight part
We know that "Riddle" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case riddle = permeate.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 PERMEATE mean?
PERMEATE noun- A watery by-product of milk production.
- Liquid that has passed through a filtration system.
- To pass through the pores or interstices of; to penetrate and pass through without causing rupture or displacement; applied especially to fluids which pass through substances of loose texture
- To enter and spread through; to pervade.