Part of a shoe against current unit (4)
I believe the answer is:
vamp
'part of a shoe' is the definition.
(I know that upper is a is part of of vamp)
'against current unit' is the wordplay.
'against' is a charade indicator (letters next to each other).
'current' becomes 'amp' (electricity).
'unit' becomes 'v' (V is an example).
'amp' put after 'v' is 'VAMP'.
(Other definitions for vamp that I've seen before include "Playing, improvise" , "Blatantly try to attract a man" , "Jazzy passage - temptress" , "Femme fatale" , "sexy woman" .)