Some garden's part is excellent (4)
I believe the answer is:
mega
'excellent' is the definition.
(old-fashioned UK slang for 'very good')
'some garden's part' is the wordplay.
'part' says the answer is hidden in the clue.
'MEGA' is hidden within 'some gardens'.
'is' is the link.
(Other definitions for mega that I've seen before include "Terrific" , "extremely good" , "wicked" , "Great American" , "'Excellent, colloquially (4)'" .)