Poison from mountaintop's freak of nature (6)
I believe the answer is:
mutant
'freak of nature' is the definition.
'mutant' can be an answer for 'freak' (thesaurus). I'm unsure of the 'of nature' bit.
'poison from mountaintop's' is the wordplay.
'from' suggests deleting specific letters.
'mountaintops' with 'poison' removed is 'MUTANT'.
Can you help me to learn more?
(Other definitions for mutant that I've seen before include "Cell that carries a mutation of a gene" , "Living thing that has undergone genetic change" , "badly bred" , "A product of genetic change" , "Living thing that undergoes genetic change" .)