Inform everyone by letter (5)
I believe the answer is:
teach
'inform' is the definition.
(teaching is a kind of informing)
'everyone by letter' is the wordplay.
'everyone' becomes 'each' (I've seen this in other clues).
'by' means one lot of letters go next to another.
'letter' becomes 't' (**).
'each' put after 't' is 'TEACH'.
(Other definitions for teach that I've seen before include "Impart knowledge" , "Pass on knowledge" , "Educate 4 across" , "Instruct, enlighten" , "Work with pupils" .)