Some drear Gothic slang (5)
I believe the answer is:
argot
'slang' is the definition.
('argot' can be a synonym of 'slang')
'some drear gothic' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'ARGOT' is hidden within 'drear gothic'.
(Other definitions for argot that I've seen before include "Jargon - patois" , "Specialised language" , "Jargon or slang (of a group)" , "Patois for groat" , "Slang for a groat (5)" .)