Language of unoriginal poetry (4)
I believe the answer is:
erse
'language' is the definition.
(historical term for Scottish Gaelic)
'unoriginal poetry' is the wordplay.
'unoriginal' means to remove the first letter (original (starting) letter removed).
'poetry' becomes 'verse' (synonyms).
'verse' with its first letter taken off is 'ERSE'.
'of' is the link.
(Other definitions for erse that I've seen before include "Possibly the language" , "Old Celtic language" , "Irish Gaelic language" , "Old Scots or Irish Gaelic" , "Old language spoken by strange seer" .)