Having reformed a school, Philip resigned (13)

I believe the answer is:

philosophical

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'resigned' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?

'having reformed a school philip' is the wordplay.
'a school philip' anagrams to 'PHILOSOPHICAL'.
But, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for philosophical that I've seen before include "Serene in the face of adversity" , "Stoical in defeat" , "Uncomplaining in defeat" , "Rationally calm" , "Calmly uncomplaining" .)

I've seen this clue in The Telegraph.

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