A very hard worker, the letter said (3)
I believe the answer is:
bee
'a very hard worker' is the definition.
'bee' can be an answer for 'worker' (a worker bee). I'm not certain of the 'a very hard' bit.
'letter said' is the wordplay.
'letter' becomes 'b'.
'said' shows a homophone (sound like).
'b' is a homophone of 'BEE'.
'the' is the link.
Can you help me to learn more?
(Other definitions for bee that I've seen before include "Honey producer" , "Flying insect (often busy)" , "Busy buzzer" , "Hymenopterous insect" , "Honey-making insect" .)