Flower power's last hour on popular drug (5)
I believe the answer is:
rhine
'flower' is the definition.
(River Rhine, something which flows)
'power's last hour on popular drug' is the wordplay.
'last' suggests the final letters.
'hour' becomes 'h'.
'on' is a charade indicator (letters next to each other).
'popular' becomes 'in' ('in' can mean trendy).
'drug' becomes 'e' (E is another name for ecstasy).
The last letter of 'power' is 'r'.
'r'+'h'+'in'+'e'='RHINE'
(Other definitions for rhine that I've seen before include "valley" , "Great German river with Lorelei Rock" , "Flower" , "River rising in Alps, with mouth at Rotterdam" , "water" .)