Language used in Alsten or Senja (5)
I believe the answer is:
norse
'language' is the definition.
(Norse is an example)
'in alsten or senja' is the wordplay.
'in' indicates a hidden word.
'NORSE' can be found hidden inside 'alsten or senja'.
'used' acts as a link.
(Other definitions for norse that I've seen before include "Old form of Norwegian" , "Native to Scandinavia" , "Viking" , "These men were Vikings" , "Ancient Scandinavian language" .)