Horse in barn again (3)
I believe the answer is:
nag
'horse' is the definition.
(term for an old horse)
'in barn again' is the wordplay.
'in' indicates a hidden word.
'NAG' can be found hidden inside 'barn again'.
(Other definitions for nag that I've seen before include "Worry constantly" , "Old or over-worked horse" , "Keep worrying" , "Hack" , "Constant worrier" .)