Press on at faulty switch (9)
I believe the answer is:
transpose
'switch' is the definition.
(to transpose two things is to switch them around)
'press on at faulty' is the wordplay.
'faulty' is an anagram indicator.
'press'+'on'+'at'='pressonat'
'pressonat' with letters rearranged gives 'TRANSPOSE'.
(Other definitions for transpose that I've seen before include "7 down (anag)" , "play in different key" , "show skill at keyboard?" , "Change the order or arrangement of" , "Cause to swap places" .)