Foremost of botanists, say, shown with prize flower (9)
I believe the answer is:
buttercup
'prize flower' is the definition.
(type of flower)
'foremost of botanists say shown' is the wordplay.
'foremost of' suggests taking the first letters.
'say' becomes 'utter' (to utter something is to speak or say it).
'shown' becomes 'cup' (I can't explain this - if you can you should give a lot more credence to this answer).
The first letter of 'botanists' is 'b'.
'b'+'utter'+'cup'='BUTTERCUP'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for buttercup that I've seen before include "Dockside seller" , "Rich yellow wild flower" , "Goldilocks, possibly" , "growth in the field" , "Common yellow meadow plant" .)