Iris Murdoch's distinctive doctrine (3)

I believe the answer is:

ism

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'distinctive doctrine' is the definition.
(suffix used in names of ideologies or doctrines)

'iris murdoch's' is the wordplay.
'ISM' is hidden within the letters.
However, I'm not sure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for ism that I've seen before include "Distinctive theory" , "Ideological movement" , "Belief system" , "Suffix denoting doctrine or practice" , "form of belief?" .)

I've seen this clue in The Telegraph.

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