Internet connection device (5)
'internet connection device' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for modem that I've seen before include "Device to connect computers by a telephone line" , "Unit connecting computer to phone line" , "Computer interface" , "Online connection device" , "Computer communication device" .)
(Other definitions for router that I've seen before include "Device that forwards data between computer networks" , "that easily beats the competition" , "Emphatic winner" , "device fitted to computer" , "Carpenter's power tool; data forwarder" .)