Teacher stuck in unfinished physics lab becomes increasingly irritable (8)

I believe the answer is:

crabbier

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'increasingly irritable' is the definition.
I can't tell whether this definition defines the answer.

'teacher stuck in unfinished physics lab' is the wordplay.
'teacher' becomes 'rabbi' (Jewish teacher).
'stuck in' is an insertion indicator.
'unfinished physics lab' becomes 'cer' (I can't justify this - if you can you should give a lot more credence to this answer).
'rabbi' placed inside 'cer' is 'CRABBIER'.

'becomes' acts as a link.

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I've seen this clue in The Guardian.

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