Setter on stream makes big bloomer (9)
I believe the answer is:
sunflower
'bloomer' is the definition.
(type of flower)
'setter on stream makes big' is the wordplay.
'setter' becomes 'sun' (the sun sets every day).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'stream makes big' becomes 'flower' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sun'+'flower'='SUNFLOWER'
Can you help me to learn more?
(Other definitions for sunflower that I've seen before include "Source of oil" , "that Van Gogh used to paint" , "Tall plant" , "Very tall bloom whose seeds are used to make margarine" , "Large golden bloom" .)