Lethal acid thrown in endless sermon (9)
I believe the answer is:
homicidal
'lethal' is the definition.
(I've seen this before)
'acid thrown in endless sermon' is the wordplay.
'thrown' indicates an anagram.
'in' means one lot of letters goes inside another.
'endless' means to remove the last letter.
'sermon' becomes 'homily' (homily is a kind of sermon).
'acid' with letters rearranged gives 'cida'.
'homily' with its final letter removed is 'homil'.
'cida' put into 'homil' is 'HOMICIDAL'.
(Other definitions for homicidal that I've seen before include "Wanting to kill someone else?" , "Lethal" , "out for blood" , "Murderous" , "likely to kill" .)