The most common and most likely answer to this crossword clue is the 7 letter word THIRDLY
I've seen this crossword clue in The Daily Telegraph
T
H
I
R
D
L
Y
Buddy explains!
The straight part
We know that "as French hens appear at Christmas?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case as french hens appear at christmas? = thirdly.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 THIRDLY mean?
THIRDLY adverb- In the third place; third in a row.