Stop working because of line in operating system (4,5)
I believe the answer is:
down tools
'stop working' is the definition.
(to down tools is to stop what you are working on)
'because of line in operating system' is the wordplay.
'because of' becomes 'downto' ('down to' can mean 'because of').
'line' becomes 'l'.
'in' indicates putting letters inside.
'operating system' becomes 'os' ('OS' can be a synonym of 'operating system').
'l' inserted within 'os' is 'ols'.
'downto'+'ols'='DOWN TOOLS'
(Other definitions for down tools that I've seen before include "go on strike" , "Stop work; strike" , "Take industrial action" .)