The most common and most likely answer to this crossword clue is the 7 plus 6 letter word EXPRESS PARCEL
I've seen this crossword clue in The Daily Telegraph
E
X
P
R
E
S
S
P
A
R
C
E
L
Buddy explains!
The straight part
We know that "package sent by special messenger" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case package sent by special messenger = express parcel.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.