Cult rabidly possessive of extremist (5)

I believe the answer is:

ultra

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'extremist' is the definition.
(ultra can mean an extremist or radical)

'cult rabidly possessive' is the wordplay.
'ULTRA' is hidden in amongst the letters.
But, I am uncertain how this is indicated.

'of' acts as a link.

Can you help me to learn more?

(Other definitions for ultra that I've seen before include "Extreme (eg in fashion or politics)" , "Person holding extreme views" , "Extreme - extremist" , "(As prefix) beyond or extreme" , "Immoderate" .)

I've seen this clue in The Telegraph.

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