Tireless passion? (3)

I believe the answer is:

ire

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'passion?' is the definition.
(ire is passionate anger)

'tireless' is the wordplay.
'IRE' is hidden in amongst the letters.
But, I am unsure how this is indicated.

Can you help me to learn more?

(Other definitions for ire that I've seen before include "Losing head" , "Strong emotion" , "heat" , "'Anger, wrath (3)'" , "Displeasure" .)

I've seen this clue in the Evening Standard.

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