Acrobat's wire (9)
I believe the answer is:
tightrope
'acrobat's wire' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tightrope that I've seen before include "Acrobat's walkway" , "High wire" , "Difficult thing to walk on" , "Acrobat's wire" , "An acrobat walks on this high above the ground" .)