Surge: this does at regular intervals! (4)
I believe the answer is:
tide
'surge' is the definition.
(I know that surge can be written as tide)
'this does at regular intervals' is the wordplay.
'at regular intervals' indicates alternate letters.
'this'+'does'='thisdoes'
The alternate letters of 'thisdoes' are 'TIDE'.
(Other definitions for tide that I've seen before include "Moon-driven sea movement" , "Periodic water movement" , "Season; current" , "One thing that waits for no man" , "In the main, movement" .)