Method of study? Drop core technical subjects (6)

I believe the answer is:

system

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'method' is the definition.
(I know that system is a type of method)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for system that I've seen before include "Plan of action" , "Organised whole made up of related parts" , "Set of things working together" , "My sets (anag.)" , "Body of doctrine - method of procedure" .)

I've seen this clue in The Guardian.

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