Solemnly promise to utter obscenities (5)

I believe the answer is:

swear

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'solemnly promise' is the definition.
(I've seen this in another clue)

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(Other definitions for swear that I've seen before include "Vow; blaspheme" , "Vow, attest" , "Emit oath or take vow" , "Promise on oath" , "Express annoyance" .)

I've seen this clue in The Sun.

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