It's supposed to be kept in Bulawayo (3)

I believe the answer is:

law

I'm a little stuck... Click here to teach me more about this clue!

'it's supposed to be' is the definition.
I can't tell whether this defines the answer.

'in bulawayo' is the wordplay.
'in' indicates the answer is hidden within the clue.
'LAW' is hidden within 'bulawayo'.

'kept' is the link.

Can you help me to learn more?

(Other definitions for law that I've seen before include "Rule, statute" , "Legislation" , "Rule prohibiting certain actions" , "Collection of rules imposed by authority" , "Official rule" .)

I've seen this clue in the Evening Standard.

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