One who deals with a hive of activity (9)
I believe the answer is:
beekeeper
'one who deals' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a hive of activity' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is located in the answer.
This explanation may well be incorrect...
'with' acts as a link.
(Other definitions for beekeeper that I've seen before include "There may be a buzz about him" , "Hive tender" , "He looks after the apiary" , "Person in charge of apiary" , "Honey farmer" .)