Monitor (6)
(Other definitions for screen that I've seen before include "Filter the mobile partition" , "Partition; test suitability" , "Cinema feature" , "Protect - project" , "Shield - broadcast" .)
'monitor' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for watchdog that I've seen before include "Guardian of people's rights, of canine origin" , "Organisation guarding against inefficient or illegal practices" , "TV show" , "Monitoring organisation" , "Monitoring body" .)