Bill is enthralled by extremely hardy and unusually thin flower (8)
I believe the answer is:
hyacinth
'flower' is the definition.
(thesaurus)
'bill is enthralled by extremely hardy and unusually thin' is the wordplay.
'bill' becomes 'ac'.
'is enthralled by' indicates putting letters inside ('enthral' can mean 'capture').
'extremely' suggests removing the centre.
'and' says to put letters next to each other.
'unusually' indicates an anagram.
'hardy' with its centre taken out is 'hy'.
'thin' is an anagram of 'inth'.
'hy'+'inth'='hyinth'
'ac' put into 'hyinth' is 'HYACINTH'.
(Other definitions for hyacinth that I've seen before include "Spring-flowering bulb" , "Girl" , "Fragrant bulbous plant" , "Fragrantly-flowered bulb" , "Bulbous flower" .)