Nocturnal phantom could be deadly in the open (10)
I believe the answer is:
nightshade
'be deadly in the open' is the definition.
I can't judge whether this definition defines the answer.
'nocturnal phantom' is the wordplay.
'nocturnal' becomes 'night' (I can't justify this - if you can you should believe this answer much more).
'phantom' becomes 'shade' (shade is a kind of phantom).
'night'+'shade'='NIGHTSHADE'
'could' acts as a link.
Can you help me to learn more?
(Other definitions for nightshade that I've seen before include "protection from the moon's brightness?" , "Belladonna" , "See 3" , "Poisonous plant" , "Plant, maybe deadly" .)