Dog or sow had run amok (6)
I believe the answer is:
shadow
'dog' is the definition.
(thesaurus)
'sow had run amok' is the wordplay.
'run amok' is an anagram indicator.
'sow'+'had'='sowhad'
'sowhad' is an anagram of 'SHADOW'.
'or' acts as a link.
(Other definitions for shadow that I've seen before include "This opposition cabinet parallels the government" , "It's cast by something blocking light" , "Trace - dog" , "Inseparable companion; dark patch" , "Reflected image" .)