Flower occasionally charms
I believe the answer is:
cam
'flower' is the definition.
(river in Cambridge, England)
'occasionally charms' is the wordplay.
'occasionally' indicates alternate letters.
The alternate letters of 'charms' are 'CAM'.
(Other definitions for cam that I've seen before include "River also called Granta" , "East Anglian river" , "English river" , "Shaft on a rotating part of machine" , "University city's river" .)