Runs external computer device (6)
I believe the answer is:
'computer device' is the definition.
(device in a computer network)
'runs external' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'external' becomes 'outer' (similar in meaning).
(Other definitions for router that I've seen before include "device fitted to computer" , "that easily beats the competition" , "Carpentry power tool" , "Device that forwards data between computer networks" , "Computer equipment" .)