Usual big green space or much smaller one (6,2,6)

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6,2,6

common or garden

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'usual' is the definition.

'big green space or much smaller one' is the wordplay.
'big green space' becomes 'common'.
'much smaller one' becomes 'garden' (I can't justify this - if you can you should believe this answer much more).
'common'+'or'+'garden'='COMMON-OR-GARDEN'

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(Other definitions for common or garden that I've seen before include "words associated with market" , "Familiar" , "usual type" , "Unexceptional" , "Ordinary" .)

I've seen this clue in The Guardian.

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