Small garden area, plain with patches of colour (8)
I believe the answer is:
skewbald
'with patches of colour' is the definition.
(as in a skewbald horse)
'small garden area plain' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'garden area' becomes 'Kew' (Kew Gardens in London).
'plain' becomes 'bald' (both can mean unadorned).
's'+'kew'+'bald'='SKEWBALD'
(Other definitions for skewbald that I've seen before include "Bicoloured creature" , "(Horse) with white and coloured marks" , "Marked with patches of white and a colour" , "maybe brown and white" , "patchy" .)