Composition some charlatan ostensibly rejected (6)
I believe the answer is:
sonata
'composition' is the definition.
(thesaurus)
'some charlatan ostensibly rejected' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'rejected' says the letters should be written in reverse.
'atanos' can be found hidden inside 'charlatan ostensibly'.
'atanos' back-to-front is 'SONATA'.
(Other definitions for sonata that I've seen before include "Solo instrumental composition" , "Sat on a kind of composition" , "Eg, Beethoven's Kreutzer or Spring" , "Composition for 1 or 2 instruments, with 3 or 4 movements" , "Musical piece" .)