Oily skin secretion (5)
I believe the answer is:
sebum
'oily skin secretion' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sebum that I've seen before include "Oily skin secretion" , "Oil on skin and hair" , "oily stuff" , "which will cover the skin" , "Lubricant" .)