Show how the devil treats badly (11)
I believe the answer is:
demonstrate
'show how' is the definition.
(thesaurus)
'devil treats badly' is the wordplay.
'devil' becomes 'demon' (synonyms).
'badly' is an anagram indicator (letters in the wrong order).
'treats' with letters rearranged gives 'strate'.
'demon'+'strate'='DEMONSTRATE'
'the' is the link.
(Other definitions for demonstrate that I've seen before include "Prove - protest" , "March" , "Make a public protest" , "Present" , "Take part in public protest" .)