Flower garden designed to be covered in loose hay (9)
I believe the answer is:
hydrangea
'flower' is the definition.
(thesaurus)
'garden designed to be covered in loose hay' is the wordplay.
'designed' indicates an anagram (designed with the letters).
'to be covered in' means one lot of letters goes inside another.
'loose' indicates anagramming the letters (letters are loosened from their original positions).
'garden' anagrammed gives 'drange'.
'hay' with letters rearranged gives 'hya'.
'drange' placed inside 'hya' is 'HYDRANGEA'.
(Other definitions for hydrangea that I've seen before include "Flower you would find in garden hay" , "Showy flower" , "Common garden plant" , "Flowering plant - angry head (anag)" , "Plant's" .)