Snapper in river a long time (6)
I believe the answer is:
camera
'snapper' is the definition.
(cameras go snap)
'river a long time' is the wordplay.
'river' becomes 'cam' (river in Cambridge, England).
'a long time' becomes 'era' (a long period of time).
'cam'+'era'='CAMERA'
'in' is the link.
(Other definitions for camera that I've seen before include "Photo-taker" , "An image-capturing device" , "Bringing equipment for producing pictures" , "Photographer's apparatus" , "source of shots?" .)