Lethal error I overlooked - ingesting astatine (5)
I believe the answer is:
fatal
'lethal' is the definition.
('fatal' can be similar in meaning to 'lethal')
'error i overlooked ingesting astatine' is the wordplay.
'error' becomes 'fail'.
'overlooked' indicates named letters should be taken away.
'ingesting' is an insertion indicator.
'astatine' becomes 'at' ('At' can be a synonym of 'astatine').
'fail' with 'i' taken out is 'fal'.
'fal' enclosing 'at' is 'FATAL'.
(Other definitions for fatal that I've seen before include "Death-dealing" , "Resulting in disaster or death" , "Involving death" , "making one late" , "Pernicious" .)