Fancy garden has to contain unknown shrubs (10)
I believe the answer is:
hydrangeas
'shrubs' is the definition.
(hydrangea is a kind of shrub)
'fancy garden has to contain unknown' is the wordplay.
'fancy' is an anagram indicator.
'to contain' indicates putting letters inside.
'unknown' becomes 'y' (algebra).
'garden'+'has'='gardenhas'
'gardenhas' is an anagram of 'hdrangeas'.
'hdrangeas' going around 'y' is 'HYDRANGEAS'.
(Other definitions for hydrangeas that I've seen before include "Garden shrubs" , "Flowering plants; ashy garden (anag.)" .)