Very extensive garden's first seen in shade (4)
I believe the answer is:
huge
'very extensive' is the definition.
'huge' can be an answer for 'extensive' (I've seen this in another clue). I'm not certain of the 'very' bit.
'garden's first seen in shade' is the wordplay.
'garden's first' becomes 'g' (hst letter of 'garden').
'seen in' is an insertion indicator.
'shade' becomes 'hue' (both can mean a colour).
'g' going within 'hue' is 'HUGE'.
Can you help me to learn more?
(Other definitions for huge that I've seen before include "Colossal" , "Vast; immense" , "Gigantic" , "Great in size or amount" , "Extremely large" .)