Examiner becomes more irritated when I get involved (6)

I believe the answer is:

tester

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'examiner' is the definition.
(I know that examiner can be written as tester)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for tester that I've seen before include "One who tries out - canopy" , "One examining" , "Spray" , "Canopy over mattress or an examiner" , "One sampling; four-poster bed canopy" .)

I've seen this clue in The Irish Times.

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