Had injected into pig's tail (6)
I believe the answer is:
shadow
'tail' is the definition.
('shadow' can be a synonym of 'tail')
'had injected into pig's' is the wordplay.
'injected into' indicates putting letters inside.
'pig' becomes 'sow' (I've seen this before).
'had' put within 'sow' is 'SHADOW'.
(Other definitions for shadow that I've seen before include "Hint, remnant; follow secretly" , "It's cast by something blocking light" , "Secretly tail someone" , "Follow closely; pale imitation" , "Dark area, weak remnant (of something)" .)