Slippery customer getting some free electricity (3)
I believe the answer is:
eel
'slippery customer' is the definition.
(eel can mean a slippery person)
'some free electricity' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'EEL' can be found hidden inside 'free electricity'.
'getting' is the link.
(Other definitions for eel that I've seen before include "Serpentine fish" , "A sea snake" , "Proverbially slippery fish" , "User of electric organ" , "Ribbon-like fish" .)