The most common and most likely answer to this crossword clue is the 9 plus 10 letter word GREETINGS (MEETINGS)
I've seen this crossword clue in The Guardian
G
R
E
E
T
I
N
G
S
(
M
E
E
T
I
N
G
S
)
Buddy explains!
The straight part
We know that "Welcome" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case welcome = greetings (meetings).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.