Her arrangement includes popular flower (5)
I believe the answer is:
rhine
'flower' is the definition.
(River Rhine, something which flows)
'her arrangement includes popular' is the wordplay.
'arrangement' indicates anagramming the letters.
'includes' is an insertion indicator.
'popular' becomes 'in' ('in' can mean trendy).
'her' is an anagram of 'rhe'.
'rhe' placed around 'in' is 'RHINE'.
(Other definitions for rhine that I've seen before include "Great German river with Lorelei Rock" , "valley" , "River of Cologne" , "Great river in Germany with Lorelei Rock" , "water" .)