Go again at the double! (6)

I believe the answer is:

repair

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

'again at the double' is the wordplay.
'again' becomes 're' (prefix meaning 'again').
'at the double' becomes 'pair' (I can't explain this - if you can you should give a lot more credence to this answer).
're'+'pair'='REPAIR'

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(Other definitions for repair that I've seen before include "Restore after damage" , "Restore; go" , "Restore to working order" , "remedy" , "Patch up" .)

I've seen this clue in the Evening Standard.

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