They may be tied up with postal deliveries (7)
I believe the answer is:
parcels
This clue might be a double definition.
'they' is the first definition.
Both the definition and answer are plural nouns.
Maybe there's an association between them I don't understand?
'tied up with postal deliveries' is the second definition.
The answer and definition can be both acts as well as being plural nouns.
Perhaps there's an association between them I don't understand?
'may be' is the link.
Can you help me to learn more?
(Another definition for parcels that I've seen is " Packages".)