Neptunian feature regularly seen that spews lava (4)
I believe the answer is:
etna
'neptunian feature' is the definition.
'etna' can be an answer for 'feature' (I have seen 'Feature of Sicily' mean 'etna' so perhaps 'feature' could also mean 'etna'). I am not sure about the 'neptunian' bit.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for etna that I've seen before include "Highest active volcano in Europe" , "Volcanic mount in Sicily" , "Highest peak in Italy south of the Alps" , "hole in the crust" , "Sicily's 15" .)