Nobody discovered poison on envelope (4)
I believe the answer is:
none
'nobody' is the definition.
(I've seen this before)
'discovered poison on envelope' is the wordplay.
'discovered' indicates the answer is hidden within the clue (remove the letters covering the answer in the clue).
'NONE' can be found hidden inside 'poison on envelope'.
(Other definitions for none that I've seen before include "0?" , "Pet menu chez Hubbard" , "office" , "Not a single person" , "Bugger all" .)