Tail the animal had caught in (6)
I believe the answer is:
Here is my best explanation:
'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 "Opposer", "Tail", "Follow; dark area", "Air of gloom", "Follow; partial darkness", "opposing counterpart", "Image caused by obscuring of light", "Dark area - follow secretly", "Trace - dog" and "Follow closely; pale imitation".)