Had pig outside to follow (6)
I believe the answer is:
shadow
'to follow' is the definition.
(shadow can mean to follow secretly)
'had pig outside' is the wordplay.
'pig' becomes 'sow' (I've seen this before).
'outside' indicates putting letters inside (some letters go outside others).
'had' placed inside 'sow' is 'SHADOW'.
(Other definitions for shadow that I've seen before include "This opposition cabinet parallels the government" , "Trail" , "opposing counterpart" , "Image cast - dog" , "Hint, remnant; follow secretly" .)