Tail the animal had caught in (6)
I believe the answer is:
shadow
'tail' is the definition.
('shadow' can be a synonym of 'tail')
'the animal had caught in' is the wordplay.
'the animal' becomes 'sow'.
'caught in' is an insertion indicator.
'sow' placed around 'had' is 'SHADOW'.
(Other definitions for shadow that I've seen before include "Trace - dog" , "Image caused by obscuring of light" , "Dark shape (on the Opposition front bench?)" , "Unilluminated area" , "Darkness caused by obstruction" .)