Use evil - terribly slippery (7)
I believe the answer is:
elusive
'slippery' is the definition.
(I've seen this before)
'use evil terribly' is the wordplay.
'terribly' indicates anagramming the letters.
'use'+'evil'='useevil'
'useevil' is an anagram of 'ELUSIVE'.
(Other definitions for elusive that I've seen before include "Hard to get hold of" , "Difficult to find or catch" , "Difficult to catch, hard to pin down" , "Slippery, hard to catch" , "Hard to grasp" .)