Loathsome brother an annoyance (9)

I believe the answer is:

abhorrent

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'loathsome' is the definition.
(I've seen this before)

'brother an annoyance' is the wordplay.
'brother an' anagrams to 'ABHORRENT'.
However, I am not sure how this is indicated.

Can you help me to learn more?

(Other definitions for abhorrent that I've seen before include "Hated" , "Disgusting, loathsome" , "hateful" , "Detestable, really off-putting" , "Inspiring disgust and loathing" .)

I've seen this clue in The Irish Times.

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