Composed sonatina on city (3,7)
I believe the answer is:
san antonio
'city' is the definition.
(San Antonio is an example)
'composed sonatina on' is the wordplay.
'composed' indicates anagramming the letters.
'sonatina'+'on'='sonatinaon'
'sonatinaon' is an anagram of 'SAN ANTONIO'.
(Other definitions for san antonio that I've seen before include "Texan mission location" , "Texan city, home to the Alamo" , "city in the New World" .)