Tail the animal had caught in (6)
I believe the answer is:
shadow
'tail' is the definition.
(synonyms)
'animal had caught in' is the wordplay.
'animal' becomes 'sow' (female pig).
'caught in' indicates putting letters inside.
'sow' enclosing 'had' is 'SHADOW'.
'the' is the link.
(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" .)