Moonlight, say, really a result of sunlight reflected (6)

I believe the answer is:
sonata
'moonlight' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'really a result of sunlight reflected' is the wordplay.
'really' becomes 'so' (both can mean 'extremely').
'a result of sunlight' becomes 'atan' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'reflected' is a reversal indicator (letters reflected back).
'atan' reversed gives 'nata'.
'so'+'nata'='SONATA'
'say' acts as a link.
Can you help me to learn more?
(Other definitions for sonata that I've seen before include "See 6" , "Beethoven's 'Moonlight', for example" , "composition for piano, perhaps" , "Instrumental work" , "just for one or two players?" .)
