Something harmful can emerge from feldspar — as it evaporates (8)
I believe the answer is:
parasite
'something harmful' is the definition.
'can emerge from feldspar as it evaporates' is the wordplay.
'can emerge from' says the answer is hidden in the clue (the answer comes out of the clue).
'PARASITE' is hidden within 'feldspar as it evaporates'.
(Other definitions for parasite that I've seen before include "louse, say" , "unwanted guest" , "Animal that feeds off another" , "Organism that lives on another" , "It lives off its host" .)