Such aid may be ‘brief’ (5)

I believe the answer is:

legal

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'brief' is the definition.
('legal brief' can be a synonym of 'brief')

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for legal that I've seen before include "It's acceptable" , "Rightful" , "Relating to jurisprudence" , "Lawful" , "Referring to the law" .)

I've seen this clue in the Evening Standard.

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