Strike idiots on behind (4,5)
I believe the answer is:
down tools
'strike' is the definition.
(to down tools is to stop working and strike)
'idiots on behind' is the wordplay.
'idiots' becomes 'tools' (tool can mean a stupid or annoying person).
'on' says to put letters next to each other.
'behind' becomes 'down' (similar in meaning).
'tools' after 'down' is 'DOWN TOOLS'.
(Other definitions for down tools that I've seen before include "Take industrial action" , "go on strike" , "Stop work; strike" .)