Mellow sound made by farm animal in south (6)
I believe the answer is:
smooth
'mellow' is the definition.
(thesaurus)
'sound made by farm animal in south' is the wordplay.
'sound made by farm animal' becomes 'moo' (sound made by a cow).
'in' means one lot of letters goes inside another.
'south' becomes 'sth' (abbreviation for south).
'moo' inserted into 'sth' is 'SMOOTH'.
(Other definitions for smooth that I've seen before include "Do ironing" , "Glossy" , "Free from roughness, bumps etc." , "Not uneven or rough" , "Calm" .)