In favour of writing music annually! (5)

I believe the answer is:

proms

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'music annually' is the definition.
I can't judge whether this definition defines the answer.

'in favour of writing' is the wordplay.
'in favour of' becomes 'pro' (being pro a particular cause).
'writing' becomes 'MS' (abbreviation for manuscript).
'pro'+'ms'='PROMS'

Can you help me to learn more?

(Other definitions for proms that I've seen before include "Henry Wood concerts" , "London summer music series" , "World's largest classical music festival, held annually (inf)" , "Sir Henry Wood's creation" , "London concerts" .)

I've seen this clue in the Evening Standard.

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