Mount regularly beaten, nag (4)
I believe the answer is:
etna
'mount' is the definition.
(Etna is an example)
'regularly beaten nag' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'beaten'+'nag'='beatennag'
The alternate letters of 'beatennag' are 'ETNA'.
(Other definitions for etna that I've seen before include "From a dangerous height" , "it may produce steam and ash" , "Sicilian hothead?" , "Long-term smoker" , "Volcano; reverse of stake" .)