Ace girl arranged cool flower (7)
I believe the answer is:
glacier
'cool flower' is the definition.
'glacier' can be an answer for 'flower' (I have seen 'River of ice' mean 'glacier' so perhaps 'river' could also mean 'glacier'). I am unsure of the 'cool' bit.
'ace girl arranged' is the wordplay.
'arranged' is an anagram indicator.
'ace'+'girl'='acegirl'
'acegirl' with letters rearranged gives 'GLACIER'.
Can you help me to learn more?
(Other definitions for glacier that I've seen before include "Slow-moving flow of ice" , "Large flow of ice" , "River of ice" , "Slow-moving mass of ice" , "Creeping ice mass" .)