Stood by US ex-president, one making bloomers (8)

I believe the answer is:

rosebush

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'one making bloomers' is the definition.
The definition suggests a singular noun which matches the answer.

'stood by us ex-president' is the wordplay.
'stood' becomes 'rose' (rose up).
'by us ex-president' becomes 'bush' (I can't justify this - if you can you should give a lot more credence to this answer).
'rose'+'bush'='ROSEBUSH'

Can you help me to learn more?

(Other definitions for rosebush that I've seen before include "Thorny growth" , "Flower supplier" , "shrub" , "Plant" .)

I've seen this clue in The Guardian.

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