Offensive operation, possibly perilous! (5)

I believe the answer is:

siege

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'offensive' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps you can see a link between them that I don't see?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for siege that I've seen before include "Surrounding and blockading of town by military" , "Military operation cutting off access" , "Offensive encirclement" , "Starving-out operation" , "Military operation surrounding a town" .)

I've seen this clue in the Evening Standard.

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