Sister broke gold part of circuit (8)
I believe the answer is:
resistor
'part of circuit' is the definition.
('resistor' is part of 'circuit')
'sister broke gold' is the wordplay.
'broke' is an anagram indicator.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'sister' is an anagram of 'resist'.
'resist'+'or'='RESISTOR'
(Other definitions for resistor that I've seen before include "Electronic component" , "Electrical component" , "Electrical device" , "Circuit component" , "Manipulator of current" .)