Language is blunt, losing head (4)
I believe the answer is:
Here is my best explanation:
'language' is the definition.
(Erse is an example)
'blunt losing head' is the subsidiary indication.
'blunt' becomes 'terse'.
'losing head' means to remove the first letter.
'terse' with its first letter removed is 'erse'.
'is' is the link.
(Other definitions for erse that I've seen before include "Old Celtic language", "old language", "Old Scots or Irish Gaelic", "Old name for Celtic languages", "Celtic tongue", "Old Gaelic for the seer", "Kind of Gaelic for seer", "The language", "Old language spoken by strange seer" and "Language".)