Weaken when I am putting brace on (6)

I believe the answer is:

impair

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'weaken' is the definition.
(I've seen this in another clue)

'when i am putting brace on' is the wordplay.
'when i am putting' becomes 'im' (I can't explain this - if you can you should believe this answer much more).
'brace' becomes 'pair' (a brace is a pair of something).
'on' says to put letters next to each other.
'im'+'pair'='IMPAIR'

Can you help me to learn more?

(Other definitions for impair that I've seen before include "Hurt or handicap" , "Hobble" , "Harm" , "blunt" , "Hinder" .)

I've seen this clue in The Irish Independent.

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