US pen for livestock (6)
I believe the answer is:
corral
'us pen for livestock' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for corral that I've seen before include "Pen for cattle in the US" , "Pen for cattle - ring of waggons" , "American enclosure for horses" , "Pen for livestock (US)" , "Shorten (book)" .)