Composition in D sharp originally containing nothing unknown (8)

I believe the answer is:

rhapsody

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'composition in d' is the definition.
'rhapsody' can be an answer for 'composition' (I've seen this before). I'm not certain of the 'in d' bit.

'sharp originally containing nothing unknown' is the wordplay.
'originally' indicates an anagram.
'containing nothing' becomes 'od' (I can't explain this - if you can you should give a lot more credence to this answer).
'unknown' becomes 'y' (algebra).
'sharp' anagrammed gives 'rhaps'.
'rhaps'+'od'+'y'='RHAPSODY'

Can you help me to learn more?

(Other definitions for rhapsody that I've seen before include "Ecstatic written or romantic musical composition" , "Exalted, free-style, composition" , "Ecstatic utterance" , "Irregular and emotional musical composition" , "Pretend to be what one is not" .)

I've seen this clue in The Guardian.

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