Method possibly messy when a tin opener is involved (6)

I believe the answer is:

system

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'method' is the definition.
(system is a kind of method)

I don't understand the remainder of the clue.

(Other definitions for system that I've seen before include "Arrangement of units that function together" , "Set of interrelated parts working together" , "Organised whole made up of related parts" , "Group of interrelated elements comprising a unified whole" , "Method (for betting?)" .)

I've seen this clue in the Evening Standard.

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