Someone else has the slippery customers (4)
I believe the answer is:
eels
'slippery customers' is the definition.
(I've seen this before)
'someone else has' is the wordplay.
'has' indicates the answer is hidden within the clue.
'EELS' is hidden in the letters of 'someone else'.
'the' is the link.
(Other definitions for eels that I've seen before include "Elongated fishes" , "Conger --; jellied --" , "Snakelike fish" , "Snakes wriggling in lees" , "Slithery fish" .)