Go off in the middle of crash course (4)

I believe the answer is:

sour

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'go off' is the definition.
(thesaurus)

'middle of crash course' is the wordplay.
'middle of' indicates the central letters.
'crash' becomes 'cause to crash' (synonyms).
'course' becomes 'our' (I can't explain this - if you can you should give a lot more credence to this answer).
The centre of 'causeto' is 's'.
's'+'our'='SOUR'

'in the' is the link.

Can you help me to learn more?

(Other definitions for sour that I've seen before include "Acid-tasting" , "'Having a sharp, biting taste (4)'" , "Disparagement of something one can't have oneself" , "Having an acidic taste" , "Acid - rancid" .)

I've seen this clue in the Sydney Morning Herald.

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