Not a useless object (5)

I believe the answer is:

cause

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'not a useless object' is the definition.
I can't tell whether this definition defines the answer.

'not a useless object' is the wordplay.
'not a' becomes 'ca' (I can't justify this - if you can you should believe this answer much more).
'useless' becomes 'us' (abbreviation for unserviceable).
'object' becomes 'E' (E is an example).
'ca'+'us'+'e'='CAUSE'

Can you help me to learn more?

(Other definitions for cause that I've seen before include "Grounds" , "Genesis" , "Give rise to" , "Bring about" , "It brings about an effect" .)

I've seen this clue in the Evening Standard.

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