One lord's strange tongue (3,5)
I believe the answer is:
old norse
'tongue' is the definition.
(Old Norse is an example)
'one lord's strange' is the wordplay.
'strange' indicates anagramming the letters.
'one'+'lords'='onelords'
'onelords' anagrammed gives 'OLD NORSE'.
(Other definitions for old norse that I've seen before include "Ancient Scandinavian language" , "Extinct Germanic language" , "Medieval Scandinavian language" .)