Regularly observed Neptunian mountain (4)
I believe the answer is:
etna
'mountain' is the definition.
(Etna is an example)
'regularly observed neptunian' is the wordplay.
'regularly observed' indicates alternate letters (letters selected at regular intervals).
The alternating letters of 'neptunian' are 'ETNA'.
(Other definitions for etna that I've seen before include "It can be scorching" , "Volcano in Italy" , "Constant smoker" , "It sometimes blows its top" , "Explosive type" .)