The most common and most likely answer to this crossword clue is the 6 letter word PACKET
I've seen this crossword clue in Financial Times
P
A
C
K
E
T
Buddy explains!
The straight part
We know that "old mailboat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old mailboat = packet.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 PACKET mean?
PACKET noun- A small pack or package; a little bundle or parcel
- Originally, a vessel employed by government to convey dispatches or mails; hence, a vessel employed in conveying dispatches, mails, passengers, and goods, and having fixed days of sailing; a mail boat. Packet boat, ship, vessel (Wikipedia).
- A specimen envelope containing small, dried plants or containing parts of plants when attached to a larger sheet.
- A small fragment of data as transmitted on some types of network, notably Ethernet networks (Wikipedia).
- A plastic bag.
- A manbulge.
- A large amount of money.
- To make up into a packet or bundle.
- To send in a packet or dispatch vessel.
- To ply with a packet or dispatch boat.
- To subject to a denial-of-service attack in which a large number of data packets are sent.