Livestock holder (4)
(Other definitions for barn that I've seen before include "spacious accommodation" , "Building for farm animals" , "Big building" , "Large shed" , "Farm storehouse" .)
'livestock holder' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pen that I've seen before include "Put down" , "Writing instrument" , "Animal enclosure" , "Prison" , "dash off" .)