The most common and most likely answer to this crossword clue is the 12 letter word CONGRATULATE
I've seen this crossword clue in The Daily Telegraph
C
O
N
G
R
A
T
U
L
A
T
E
Buddy explains!
The straight part
We know that "Send best wishes to" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case send best wishes to = congratulate.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 CONGRATULATE mean?
CONGRATULATE verb- To express one’s sympathetic pleasure or joy to the person(s) it is felt for.
- To consider oneself fortunate in some matter.