Military action, say, is about capturing enemy primarily (5)
I believe the answer is:
siege
'military action' is the definition.
(siege is a kind of military action)
'say is about capturing enemy primarily' is the wordplay.
'say' becomes 'eg' (**).
'about' is a reversal indicator.
'capturing' is an insertion indicator.
'primarily' indicates taking the first letters.
The initial letter of 'enemy' is 'e'.
'eg'+'is'='egis'
'egis' backwards is 'sige'.
'sige' placed around 'e' is 'SIEGE'.
(Other definitions for siege that I've seen before include "Act of surrounding a town or fortress" , "Operation to surround and cut off supplies" , "Persistent attempt to gain" , "Persistent attack" , "Military operation to seal off (town)" .)