Touching lines (8)
I believe the answer is:
tangents
'touching lines' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tangents that I've seen before include "Straight lines - ten gnats (anag)" , "Straight lines that touch but don't intersect a curve" , "Straight lines that touche curves" , "Straight lines that touch curves" , "Touching lines" .)