Feature of many a whodunit (6)
I believe the answer is:
poison
'feature of many a whodunit' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for poison that I've seen before include "Venom, harmful substance" , "Deadly substance" , "Substance that damages tissues or causes death" , "Damaging stuff - drink?" , "Meat for someone else" .)