I think "clerk" rhymes with "clark" in most UK English accents, but this does not mean that all English people pronounce it in the same way. There is a broad north-south divide in the vowel sound. In the North, it might be something like an 'aa' or sound than a 'ar' sound. And some places will have a more 'rhotic' approach to the letter r than others.

Regarding the word 'clerk' this gives UK/US examples, using a 'received pronunciation' for the UK example.

https://dictionary.cambridge.org/pronunciation/english/clerk