Dog had to be eaten by pig (6)
I believe the answer is:
shadow
'dog' is the definition.
(thesaurus)
'had to be eaten by pig' is the wordplay.
'to be eaten by' is an insertion indicator.
'pig' becomes 'sow' (I've seen this before).
'had' going into 'sow' is 'SHADOW'.
(Other definitions for shadow that I've seen before include "Follow - dark patch" , "Image cast - dog" , "Unilluminated area" , "This opposition cabinet parallels the government" , "Dark area, weak remnant (of something)" .)