Language once spoken in Bergen or Senja (5)
I believe the answer is:
norse
'language once spoken' is the definition.
(historical language)
'in bergen or senja' is the wordplay.
'in' indicates the answer is hidden within the clue.
'NORSE' can be found hidden inside 'bergen or senja'.
(Other definitions for norse that I've seen before include "Native to Scandinavia" , "These men were Vikings" , "Old form of Norwegian" , "Native to Norway" , "Relating to ancient Scandinavian" .)