The most common and most likely answer to this crossword clue is the 5 plus 9 letter word STAMP COLLECTOR
I've seen this crossword clue in The Sunday Telegraph
S
T
A
M
P
C
O
L
L
E
C
T
O
R
More clues leading to the result STAMP COLLECTOR
Buddy explains!
The straight part
We know that "Hobbyist for whom mail brings items of interest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hobbyist for whom mail brings items of interest = stamp collector.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.