Form of industrial action (4-2-4)
(Another definition for work-to-rule that I've seen is " Industrial action".)
'form of industrial action' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for strike that I've seen before include "Work stoppage over industrial dispute" , "Hit at a trade dispute" , "Hit out in industrial dispute" , "make an impact" , "Make a sudden large financial gain" .)