I get a bit tetchy when it irritates (4)

I believe the answer is:

itch

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'it irritates' is the definition.
The definition suggests a singular noun which matches the answer.

'i get a bit tetchy' is the wordplay.
'i' becomes 'ich' (I can't justify this - if you can you should give a lot more credence to this answer).
'get' indicates putting letters inside.
'a bit' indicates taking the first letters (I've seen 'bit of' mean this).
The initial letter of 'tetchy' is 't'.
'ich' placed around 't' is 'ITCH'.

'when' acts as a link.

Can you help me to learn more?

(Other definitions for itch that I've seen before include "A nagging desire" , "Nagging wish" , "Yearn" , "Yen" , "are dying" .)

I've seen this clue in the Evening Standard.

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