Line up for walk (9)
I believe the answer is:
tightrope
'line up' is the definition.
(line that's up in the air)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for tightrope that I've seen before include "Acrobat's walkway" , "Difficult thing to walk on" , "On which acrobats walk" , "An acrobat walks on this high above the ground" , "Entertaining line" .)