One regularly dispatching letters - nearly 50 altogether (2,3)
I believe the answer is:
in all
'altogether' is the definition.
(synonyms)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for in all that I've seen before include "Taken together" , "Taken as a whole" , "Altogether" , "In total" , "Summarising" .)