The most common and most likely answer to this crossword clue is the 5 plus 5 letter word LAVER BREAD
I've seen this crossword clue in The Guardian
L
A
V
E
R
B
R
E
A
D
Buddy explains!
The straight part
We know that "Welsh vegetarian food /Welsh vegetarian food" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case welsh vegetarian food /welsh vegetarian food = laver bread.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.