Son entering big race venues speeds up (4-6)

I believe the answer is:

fast-tracks

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'venues speeds up' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'son entering big race' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'entering' indicates putting letters inside.
'big' becomes 'fat'.
'race' becomes 'tracks' (I can't explain this - if you can you should believe this answer much more).
's' placed into 'fat' is 'fast'.
'fast'+'tracks'='FAST-TRACKS'

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I've seen this clue in The Guardian.

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