Hard to catch (7)
I believe the answer is:
elusive
'hard to catch' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for elusive that I've seen before include "Not easy to pin down" , "Vile Sue, not easy to find" , "Difficult to grasp" , "Use veil to be difficult to catch" , "Hard to pin down, slippery" .)