Article in part of electrical system is decaying (9)
I believe the answer is:
withering
'is decaying' is the definition.
(wither can mean to decay or die)
'article in part of electrical system' is the wordplay.
'article' becomes 'the' (the definite article in English).
'in' is an insertion indicator.
'part of electrical system' becomes 'wiring' (electrical wiring).
'the' inserted within 'wiring' is 'WITHERING'.
(Other definitions for withering that I've seen before include "Supercilious" , "Devastating" .)