Crossword Genius

Not for the first time, shut up! (6)

Ross

I believe the answer is:

reseal

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'up' is the definition.
Although both the answer and definition are verbs in their base form, I cannot see how one could define the other.

'not for the first time shut' is the wordplay.
'not for the first time' becomes 're' (I can't explain this - if you can you should give a lot more credence to this answer).
'shut' becomes 'seal' (sealing is a kind of shutting).
're'+'seal'='RESEAL'

Can you help me to learn more?

(Other definitions for reseal that I've seen before include "Close securely again" , "Close again tightly" .)

I've seen this clue in the Evening Standard.
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