Safety device makes William's weird sister fall over (4,6)

'safety device' is the definition.
(electrical device)

'makes william's weird sister fall over' is the wordplay.
'makes' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'william's weird sister' becomes 'witch' (as in the weird sisters in Shakespeare's Macbeth).
'fall' becomes 'trip' (as in tripping over).
'over' means one lot of letters go next to another.
's'+'witch'='switch'
'switch' put after 'trip' is 'TRIP SWITCH'.