Extremely happy a garden cultivated this? (9)
I believe the answer is:
hydrangea
'this?' is the definition.
The definition suggests a singular noun which matches the answer.
'extremely happy a garden cultivated' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'cultivated' indicates an anagram.
'happy' with its middle taken out is 'hy'.
'a'+'garden'='agarden'
'agarden' anagrammed gives 'drangea'.
'hy'+'drangea'='HYDRANGEA'
(Other definitions for hydrangea that I've seen before include "This flower is strangely a hay danger" , "Garden flower - garden hay (anag)" , "Plant's" , "Flower you might find in garden hay, oddly" , "Common garden plant" .)