Encirclement, say, if enemy gets energetic at outset (5)
I believe the answer is:
siege
'encirclement' is the definition.
(siege is a kind of encirclement)
'say if enemy gets energetic at outset' is the wordplay.
'at outset' indicates taking the first letters.
The initial letters of 'say if enemy gets energetic' is 'SIEGE'.
(Other definitions for siege that I've seen before include "Persistent attempt to gain" , "Surrounding and blockading of town by military" , "Offensive encirclement" , "Operation to surround a town/fortress" , "Military operation surrounding a town" .)