Little piece of music for the youngest brother (7)

I believe the answer is:

minimus

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'brother' is the definition.
Both the answer and definition are adjectives.
Perhaps you can see an association between them that I can't see?

'little piece of music for the youngest' is the wordplay.
'little' becomes 'mini'.
'piece of' says to take the initial letters.
'for' is a charade indicator (letters next to each other).
'the youngest' becomes 'us' (I am not sure about this - if you are sure you should believe this answer much more).
The initial letter of 'music' is 'm'.
'mini'+'m'+'us'='MINIMUS'

Can you help me to learn more?

(Other definitions for minimus that I've seen before include "name for youngest son?" , "youngest family member" .)

I've seen this clue in the Evening Standard.

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