Observe siege at regular intervals (3)
I believe the answer is:
see
'observe' is the definition.
(thesaurus)
'siege at regular intervals' is the wordplay.
'at regular intervals' indicates alternate letters.
The alternating letters of 'siege' are 'SEE'.
(Other definitions for see that I've seen before include "Bishop's seat" , "Understand; watch" , "Jurisdiction of a bishop" , "make out" , "Three points" .)