Language making you gloomy? Not half (5)
I believe the answer is:
lingo
'language' is the definition.
(lingo is a kind of language)
'gloomy? not half' is the wordplay.
'gloomy?' becomes 'ling' (I am not sure about this - if you are sure you should believe this answer much more).
'not' becomes 'no' (I've seen this in other clues).
'half' suggests halving the letters.
'no' cut in half is 'o'.
'ling'+'o'='LINGO'
'making you' acts as a link.
Can you help me to learn more?
(Other definitions for lingo that I've seen before include "special words" , "Language (slang)" , "Foreign language or dialect: jargon" , "Vernacular" , "Jargon, language (colloq.)" .)