Source of poison near suspicious deaths (10)
I believe the answer is:
nightshade
'source of poison' is the definition.
(plant with poisonous berries)
'near suspicious deaths' is the wordplay.
'near' becomes 'nigh' (synonyms).
'suspicious' is an anagram indicator (an unusual or suspect spelling).
'deaths' anagrammed gives 'tshade'.
'nigh'+'tshade'='NIGHTSHADE'
(Other definitions for nightshade that I've seen before include "Belladonna" , "protection from the moon's brightness?" , "Poisonous plant" , "See 3" , "Plant, maybe deadly" .)