Be on ladder (3)
I believe the answer is:
run
I believe this is a double definition.
'be on' is the first definition.
(as in a device switched on or running)
'ladder' is the second definition.
(both can mean a tear in tights)
(Other definitions for run that I've seen before include "Hasten" , "goal for England player?" , "string of performances" , "Manage; function" , "politician must do this to get elected" .)