Ring when computer device fails to start (5)
I believe the answer is:
outer
'ring' is the definition.
(outer ring on an archery target)
'computer device fails to start' is the wordplay.
'computer device' becomes 'router' (device in a computer network).
'fails to start' means to remove the first letter.
'router' with its first letter taken away is 'OUTER'.
'when' is the link.
(Other definitions for outer that I've seen before include "Outside; external" , "Ring for double scores on a dartboard" , "Surrounding" , "Exterior or external, like space" , "On the periphery" .)