Pledge made by prisoner of hero at Hyperborea (4)

I believe the answer is:

oath

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'pledge made by prisoner' is the definition.
'oath' can be an answer for 'pledge' (I've seen this before). I'm unsure of the 'made by prisoner' bit.

'of hero at hyperborea' is the wordplay.
'of' indicates a hidden word.
'OATH' can be found hidden inside 'hero at hyperborea'.

Can you help me to learn more?

(Other definitions for oath that I've seen before include "Vow, sworn word" , "Swear word" , "Commitment to tell the truth" , "It's taken in court" , "One sworn" .)

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