Dark image swine had inserted (6)
I believe the answer is:
shadow
'dark image' is the definition.
(I've seen this before)
'swine had inserted' is the wordplay.
'swine' becomes 'sow' (sow is a kind of swine).
'inserted' is an insertion indicator.
'sow' going around 'had' is 'SHADOW'.
(Other definitions for shadow that I've seen before include "Follow; partial darkness" , "Follow secretly; obscure" , "Dark area - follow secretly" , "Dark area, weak remnant (of something)" , "Dark area from which light is blocked" .)