Language very lacking in poetry (4)
I believe the answer is:
erse
'language' is the definition.
(historical term for Scottish Gaelic)
'very lacking in poetry' is the wordplay.
'very' becomes 'v' (abbreviation).
'lacking in' indicates named letters should be taken away.
'poetry' becomes 'verse' (synonyms).
'verse' with 'v' removed is 'ERSE'.
(Other definitions for erse that I've seen before include "Irish Gaelic language" , "Old name for Celtic languages" , "Old language spoken by strange seer" , "Possibly the language" , "Old Celtic language" .)