Flower in America matters (3)
I believe the answer is:
cam
'flower' is the definition.
(river in Cambridge, England)
'in america matters' is the wordplay.
'in' says the answer is hidden in the clue.
'CAM' is hidden within 'america matters'.
(Other definitions for cam that I've seen before include "Rotating part in machine; river" , "East Anglian river" , "Part of engine" , "Shaft on a rotating part in machinery" , "Rotating mechanism" .)