Old boy restricted by disease — one gets lots of letters (2,3)
I believe the answer is:
po box
'one gets lots of letters' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'old boy restricted by disease' is the wordplay.
'old boy' becomes 'ob' (abbreviation).
'restricted by' indicates putting letters inside (I've seen this in other clues).
'disease' becomes 'pox' (pox is a kind of disease).
'ob' placed inside 'pox' is 'PO BOX'.
Can you help me to learn more?
(Another definition for po box that I've seen is " Address".)