Language used by Alexander Selkirk (4)
I believe the answer is:
erse
'language' is the definition.
(historical term for Scottish Gaelic)
'used by alexander selkirk' is the wordplay.
'used by' says the answer is hidden in the clue.
'ERSE' can be found hidden inside 'alexander selkirk'.
(Other definitions for erse that I've seen before include "Old language spoken by strange seer" , "Old Scots or Irish Gaelic" , "Old Celtic language" , "Name sometimes given to languages of the Celts in Ireland and Scotland" , "Old name for Celtic languages" .)