It emerges as a key assignment (8)

I believe the answer is:

emission

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'it emerges' is the definition.
The definition suggests a singular noun which matches the answer.

'a key assignment' is the wordplay.
'a key' becomes 'E' (a musical key).
'assignment' becomes 'mission' (mission is a kind of assignment).
'e'+'mission'='EMISSION'

'as' acts as a link.

Can you help me to learn more?

(Other definitions for emission that I've seen before include "Release" , "Something given out" , "Process of giving out" , "issue" , "Going out" .)

I've seen this clue in the Evening Standard.

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