Impressive flower up for arrangement (8)
I believe the answer is:
powerful
'impressive' is the definition.
(thesaurus)
'flower up for arrangement' is the wordplay.
'for arrangement' indicates anagramming the letters.
'flower'+'up'='flowerup'
'flowerup' is an anagram of 'POWERFUL'.
(Other definitions for powerful that I've seen before include "Pure wolf (anag.)" , "Potent, forceful" , "Dominant" , "Having a strong effect" , "Like a potentate" .)