Not planned, flat, for example (10)

I believe the answer is:

accidental

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'not planned flat' is the definition.
'accidental' can be an answer for 'flat' (thesaurus). I'm not sure about the 'not planned' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for accidental that I've seen before include "Subsidiary" , "Happening by chance" , "without planning" , "indication to modify tone slightly" , "Sign raising or lowering a musical note" .)

I've seen this clue in The Telegraph.

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