Some infernal plant (4)
I believe the answer is:
fern
'plant' is the definition.
(fern is a kind of plant)
'some infernal' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'FERN' is hidden in the letters of 'infernal'.
(Other definitions for fern that I've seen before include "Feathery-leaved plant" , "A frond is its leaf" , "Flowerless plant with feathery fronds" , "Plant with featherlike leaves" , "Fronded plant" .)