Most irritating part of time in prison? (4)

I believe the answer is:

stir

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'time in prison?' is the definition.
'stir' can be an answer for 'prison?' (informal term for prison). I am not sure about the 'time in' bit.

'most irritating part' is the wordplay.
'part' says the answer is hidden in the clue.
'STIR' can be found hidden inside 'most irritating'.

'of' acts as a link.

Can you help me to learn more?

(Other definitions for stir that I've seen before include "Get moving" , "College" , "Sensation" , "Whisk or whip" , "Whisk or rotate" .)

I've seen this clue in The Irish Times.

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