Farm animal outside possessed tail (6)
I believe the answer is:
shadow
'tail' is the definition.
(I know that tail can be written as shadow)
'farm animal outside possessed' is the wordplay.
'farm animal' becomes 'sow' (female pig).
'outside' is an insertion indicator.
'possessed' becomes 'had' (to have something is to possess it).
'sow' placed around 'had' is 'SHADOW'.
(Other definitions for shadow that I've seen before include "Darker part" , "Image caused by obscuring of light" , "Area of darkness caused by obstructing light" , "Dark area from which light is blocked" , "Sort of boxer - dog" .)