Lines known by the expert (5)
I believe the answer is:
ropes
This clue is a double definition.
'lines' is the first definition.
(rope is a kind of line)
'known by the expert' is the second definition.
(as in a phrase like 'show me the ropes')
(Other definitions for ropes that I've seen before include "Stays" , "Lashings" , "Stout cords" , "Hawsers" , "Guys" .)