Gap garment a third off — over nine grand! (7)

I believe the answer is:

opening

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'gap' is the definition.
(synonyms)

'garment a third off over nine grand' is the wordplay.
'garment a third off' becomes 'op' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'over' is a reversal indicator.
'grand' becomes 'G' (synonyms).
'nine' reversed gives 'enin'.
'op'+'enin'+'g'='OPENING'

Can you help me to learn more?

(Other definitions for opening that I've seen before include "Inauguration" , "Beginning; breach" , "Rent" , "Vent or opportunity" , "Initial" .)

I've seen this clue in The Guardian.

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