One who figures poisonously in the natural world (5)
I believe the answer is:
adder
'natural world' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'one who figures poisonously in' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'who' becomes 'dr' (from TV series Doctor Who).
'figures poisonously' becomes 'de' (I am not sure about this - if you are sure you should believe this answer much more).
'in' is an insertion indicator.
'dr' placed around 'de' is 'dder'.
'a'+'dder'='ADDER'
'the' is the link.
Can you help me to learn more?
(Other definitions for adder that I've seen before include "British snake; summer" , "Small poisonous snake, viper" , "Totting snake" , "Totter" , "Venomous type" .)