Callas adds Greek character to Mexican musical style (8)
I believe the answer is:
mariachi
'mexican musical style' is the definition.
The definition suggests a singular noun which matches the answer.
'callas adds greek character' is the wordplay.
'callas' becomes 'maria' (Maria Callas).
'adds greek' becomes 'ch' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'character' becomes 'i'.
'maria'+'ch'+'i'='MARIACHI'
'to' is the link.
Can you help me to learn more?
(Other definitions for mariachi that I've seen before include "Source of Mexican music" , "street musicians?" , "Traditional Mexican folk music" , "Mexican players" , "one who performs music" .)