The most common and most likely answer to this crossword clue is the 7 letter word P.O.BOX
I've seen this crossword clue in The Guardian
P
.
O
.
B
O
X
Buddy explains!
The straight part
We know that "Old boy restricted by disease — one gets lots of letters (2,3)/one gets lots of letters" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old boy restricted by disease — one gets lots of letters (2,3)/one gets lots of letters = p.o.box.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.