Difficult to catch (7)
I believe the answer is:
elusive
'difficult to catch' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for elusive that I've seen before include "Hard to find" , "Transient" , "Use veil to be difficult to catch" , "Evil use (anag.)" , "Vile sue is hard to catch" .)